Multipath route lookups should consider knowledge about next hops and not
select a hop that is known to be failed.

Example:

                     [h2]                   [h3]   15.0.0.5
                      |                      |
                     3|                     3|
                    [SP1]                  [SP2]--+
                     1  2                   1     2
                     |  |     /-------------+     |
                     |   \   /                    |
                     |     X                      |
                     |    / \                     |
                     |   /   \---------------\    |
                     1  2                     1   2
         12.0.0.2  [TOR1] 3-----------------3 [TOR2] 12.0.0.3
                     4                         4
                      \                       /
                        \                    /
                         \                  /
                          -------|   |-----/
                                 1   2
                                [TOR3]
                                  3|
                                   |
                                  [h1]  12.0.0.1

host h1 with IP 12.0.0.1 has 2 paths to host h3 at 15.0.0.5:

    root@h1:~# ip ro ls
    ...
    12.0.0.0/24 dev swp1  proto kernel  scope link  src 12.0.0.1
    15.0.0.0/16
            nexthop via 12.0.0.2  dev swp1 weight 1
            nexthop via 12.0.0.3  dev swp1 weight 1
    ...

If the link between tor3 and tor1 is down and the link between tor1
and tor2 then tor1 is effectively cut-off from h1. Yet the route lookups
in h1 are alternating between the 2 routes: ping 15.0.0.5 gets one and
ssh 15.0.0.5 gets the other. Connections that attempt to use the
12.0.0.2 nexthop fail since that neighbor is not reachable:

    root@h1:~# ip neigh show
    ...
    12.0.0.3 dev swp1 lladdr 00:02:00:00:00:1b REACHABLE
    12.0.0.2 dev swp1  FAILED
    ...

The failed path can be avoided by considering known neighbor information
when selecting next hops. If the neighbor lookup fails we have no
knowledge about the nexthop, so give it a shot. If there is an entry
then only select the nexthop if the state is sane. This is similar to
what fib_detect_death does.

To maintain backward compatibility use of the neighbor information is
based on a new sysctl, fib_multipath_use_neigh.
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Reviewed-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>