Fix handling IPv4 routes referencing a nexthop via its id by replacing
calls to fib_info_nh() with fib_info_nhc().

Trying to add an IPv4 route referencing a nextop via nhid:

    $ ip link set up swp5
    $ ip a a 10.0.0.1/24 dev swp5
    $ ip nexthop add dev swp5 id 20 via 10.0.0.2
    $ ip route add 10.0.1.0/24 nhid 20

triggers warnings when trying to handle the route:

[  528.805763] ------------[ cut here ]------------
[  528.810437] WARNING: CPU: 3 PID: 53 at include/net/nexthop.h:468 __prestera_fi_is_direct+0x2c/0x68 [prestera]
[  528.820434] Modules linked in: prestera_pci act_gact act_police sch_ingress cls_u32 cls_flower prestera arm64_delta_tn48m_dn_led(O) arm64_delta_tn48m_dn_cpld(O) [last unloaded: prestera_pci]
[  528.837485] CPU: 3 PID: 53 Comm: kworker/u8:3 Tainted: G           O       6.4.5 #1
[  528.845178] Hardware name: delta,tn48m-dn (DT)
[  528.849641] Workqueue: prestera_ordered __prestera_router_fib_event_work [prestera]
[  528.857352] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[  528.864347] pc : __prestera_fi_is_direct+0x2c/0x68 [prestera]
[  528.870135] lr : prestera_k_arb_fib_evt+0xb20/0xd50 [prestera]
[  528.876007] sp : ffff80000b20bc90
[  528.879336] x29: ffff80000b20bc90 x28: 0000000000000000 x27: ffff0001374d3a48
[  528.886510] x26: ffff000105604000 x25: ffff000134af8a28 x24: ffff0001374d3800
[  528.893683] x23: ffff000101c89148 x22: ffff000101c89000 x21: ffff000101c89200
[  528.900855] x20: ffff00013641fda0 x19: ffff800009d01088 x18: 0000000000000059
[  528.908027] x17: 0000000000000277 x16: 0000000000000000 x15: 0000000000000000
[  528.915198] x14: 0000000000000003 x13: 00000000000fe400 x12: 0000000000000000
[  528.922371] x11: 0000000000000002 x10: 0000000000000aa0 x9 : ffff8000013d2020
[  528.929543] x8 : 0000000000000018 x7 : 000000007b1703f8 x6 : 000000001ca72f86
[  528.936715] x5 : 0000000033399ea7 x4 : 0000000000000000 x3 : ffff0001374d3acc
[  528.943886] x2 : 0000000000000000 x1 : ffff00010200de00 x0 : ffff000134ae3f80
[  528.951058] Call trace:
[  528.953516]  __prestera_fi_is_direct+0x2c/0x68 [prestera]
[  528.958952]  __prestera_router_fib_event_work+0x100/0x158 [prestera]
[  528.965348]  process_one_work+0x208/0x488
[  528.969387]  worker_thread+0x4c/0x430
[  528.973068]  kthread+0x120/0x138
[  528.976313]  ret_from_fork+0x10/0x20
[  528.979909] ---[ end trace 0000000000000000 ]---
[  528.984998] ------------[ cut here ]------------
[  528.989645] WARNING: CPU: 3 PID: 53 at include/net/nexthop.h:468 __prestera_fi_is_direct+0x2c/0x68 [prestera]
[  528.999628] Modules linked in: prestera_pci act_gact act_police sch_ingress cls_u32 cls_flower prestera arm64_delta_tn48m_dn_led(O) arm64_delta_tn48m_dn_cpld(O) [last unloaded: prestera_pci]
[  529.016676] CPU: 3 PID: 53 Comm: kworker/u8:3 Tainted: G        W  O       6.4.5 #1
[  529.024368] Hardware name: delta,tn48m-dn (DT)
[  529.028830] Workqueue: prestera_ordered __prestera_router_fib_event_work [prestera]
[  529.036539] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[  529.043533] pc : __prestera_fi_is_direct+0x2c/0x68 [prestera]
[  529.049318] lr : __prestera_k_arb_fc_apply+0x280/0x2f8 [prestera]
[  529.055452] sp : ffff80000b20bc60
[  529.058781] x29: ffff80000b20bc60 x28: 0000000000000000 x27: ffff0001374d3a48
[  529.065953] x26: ffff000105604000 x25: ffff000134af8a28 x24: ffff0001374d3800
[  529.073126] x23: ffff000101c89148 x22: ffff000101c89148 x21: ffff00013641fda0
[  529.080299] x20: ffff000101c89000 x19: ffff000101c89020 x18: 0000000000000059
[  529.087471] x17: 0000000000000277 x16: 0000000000000000 x15: 0000000000000000
[  529.094642] x14: 0000000000000003 x13: 00000000000fe400 x12: 0000000000000000
[  529.101814] x11: 0000000000000002 x10: 0000000000000aa0 x9 : ffff8000013cee80
[  529.108985] x8 : 0000000000000018 x7 : 000000007b1703f8 x6 : 0000000000000018
[  529.116157] x5 : 00000000d3497eb6 x4 : ffff000105604081 x3 : 000000008e979557
[  529.123329] x2 : 0000000000000000 x1 : ffff00010200de00 x0 : ffff000134ae3f80
[  529.130501] Call trace:
[  529.132958]  __prestera_fi_is_direct+0x2c/0x68 [prestera]
[  529.138394]  prestera_k_arb_fib_evt+0x6b8/0xd50 [prestera]
[  529.143918]  __prestera_router_fib_event_work+0x100/0x158 [prestera]
[  529.150313]  process_one_work+0x208/0x488
[  529.154348]  worker_thread+0x4c/0x430
[  529.158030]  kthread+0x120/0x138
[  529.161274]  ret_from_fork+0x10/0x20
[  529.164867] ---[ end trace 0000000000000000 ]---

and results in a non offloaded route:

    $ ip route
    10.0.0.0/24 dev swp5 proto kernel scope link src 10.0.0.1 rt_trap
    10.0.1.0/24 nhid 20 via 10.0.0.2 dev swp5 rt_trap

When creating a route referencing a nexthop via its ID, the nexthop will
be stored in a separate nh pointer instead of the array of nexthops in
the fib_info struct. This causes issues since fib_info_nh() only handles
the nexthops array, but not the separate nh pointer, and will loudly
WARN about it.

In contrast fib_info_nhc() handles both, but returns a fib_nh_common
pointer instead of a fib_nh pointer. Luckily we only ever access fields
from the fib_nh_common parts, so we can just replace all instances of
fib_info_nh() with fib_info_nhc() and access the fields via their
fib_nh_common names.

This allows handling IPv4 routes with an external nexthop, and they now
get offloaded as expected:

    $ ip route
    10.0.0.0/24 dev swp5 proto kernel scope link src 10.0.0.1 rt_trap
    10.0.1.0/24 nhid 20 via 10.0.0.2 dev swp5 offload rt_offload

Fixes: 396b80cb ("net: marvell: prestera: Add neighbour cache accounting")
Signed-off-by: Jonas Gorski <jonas.gorski@bisdn.de>
Acked-by: Elad Nachman <enachman@marvell.com>
Link: https://lore.kernel.org/r/20230804101220.247515-1-jonas.gorski@bisdn.deSigned-off-by: Jakub Kicinski <kuba@kernel.org>