l2tp: fix configuration passed to setup_udp_tunnel_sock()

Unused fields of udp_cfg must be all zeros. Otherwise
setup_udp_tunnel_sock() fills ->gro_receive and ->gro_complete
callbacks with garbage, eventually resulting in panic when used by
udp_gro_receive().

[   72.694123] BUG: unable to handle kernel paging request at ffff880033f87d78
[   72.695518] IP: [<ffff880033f87d78>] 0xffff880033f87d78
[   72.696530] PGD 26e2067 PUD 26e3067 PMD 342ed063 PTE 8000000033f87163
[   72.696530] Oops: 0011 [#1] SMP KASAN
[   72.696530] Modules linked in: l2tp_ppp l2tp_netlink l2tp_core ip6_udp_tunnel udp_tunnel pptp gre pppox ppp_generic slhc crc32c_intel ghash_clmulni_intel jitterentropy_rng sha256_generic hmac drbg ansi_cprng aesni_intel evdev aes_x86_64 ablk_helper cryptd lrw gf128mul glue_helper serio_raw acpi_cpufreq button proc\
essor ext4 crc16 jbd2 mbcache virtio_blk virtio_net virtio_pci virtio_ring virtio
[   72.696530] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 4.7.0-rc1 #1
[   72.696530] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Debian-1.8.2-1 04/01/2014
[   72.696530] task: ffff880035b59700 ti: ffff880035b70000 task.ti: ffff880035b70000
[   72.696530] RIP: 0010:[<ffff880033f87d78>]  [<ffff880033f87d78>] 0xffff880033f87d78
[   72.696530] RSP: 0018:ffff880035f87bc0  EFLAGS: 00010246
[   72.696530] RAX: ffffed000698f996 RBX: ffff88003326b840 RCX: ffffffff814cc823
[   72.696530] RDX: ffff88003326b840 RSI: ffff880033e48038 RDI: ffff880034c7c780
[   72.696530] RBP: ffff880035f87c18 R08: 000000000000a506 R09: 0000000000000000
[   72.696530] R10: ffff880035f87b38 R11: ffff880034b9344d R12: 00000000ebfea715
[   72.696530] R13: 0000000000000000 R14: ffff880034c7c780 R15: 0000000000000000
[   72.696530] FS:  0000000000000000(0000) GS:ffff880035f80000(0000) knlGS:0000000000000000
[   72.696530] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   72.696530] CR2: ffff880033f87d78 CR3: 0000000033c98000 CR4: 00000000000406a0
[   72.696530] Stack:
[   72.696530]  ffffffff814cc834 ffff880034b93468 0000001481416818 ffff88003326b874
[   72.696530]  ffff880034c7ccb0 ffff880033e48038 ffff88003326b840 ffff880034b93462
[   72.696530]  ffff88003326b88a ffff88003326b88c ffff880034b93468 ffff880035f87c70
[   72.696530] Call Trace:
[   72.696530]  <IRQ>
[   72.696530]  [<ffffffff814cc834>] ? udp_gro_receive+0x1c6/0x1f9
[   72.696530]  [<ffffffff814ccb1c>] udp4_gro_receive+0x2b5/0x310
[   72.696530]  [<ffffffff814d989b>] inet_gro_receive+0x4a3/0x4cd
[   72.696530]  [<ffffffff81431b32>] dev_gro_receive+0x584/0x7a3
[   72.696530]  [<ffffffff810adf7a>] ? __lock_is_held+0x29/0x64
[   72.696530]  [<ffffffff814321f7>] napi_gro_receive+0x124/0x21d
[   72.696530]  [<ffffffffa000b145>] virtnet_receive+0x8df/0x8f6 [virtio_net]
[   72.696530]  [<ffffffffa000b27e>] virtnet_poll+0x1d/0x8d [virtio_net]
[   72.696530]  [<ffffffff81431350>] net_rx_action+0x15b/0x3b9
[   72.696530]  [<ffffffff815893d6>] __do_softirq+0x216/0x546
[   72.696530]  [<ffffffff81062392>] irq_exit+0x49/0xb6
[   72.696530]  [<ffffffff81588e9a>] do_IRQ+0xe2/0xfa
[   72.696530]  [<ffffffff81587a49>] common_interrupt+0x89/0x89
[   72.696530]  <EOI>
[   72.696530]  [<ffffffff810b05df>] ? trace_hardirqs_on_caller+0x229/0x270
[   72.696530]  [<ffffffff8102b3c7>] ? default_idle+0x1c/0x2d
[   72.696530]  [<ffffffff8102b3c5>] ? default_idle+0x1a/0x2d
[   72.696530]  [<ffffffff8102bb8c>] arch_cpu_idle+0xa/0xc
[   72.696530]  [<ffffffff810a6c39>] default_idle_call+0x1a/0x1c
[   72.696530]  [<ffffffff810a6d96>] cpu_startup_entry+0x15b/0x20f
[   72.696530]  [<ffffffff81039a81>] start_secondary+0x12c/0x133
[   72.696530] Code: ff ff ff ff ff ff ff ff ff ff 7f ff ff ff ff ff ff ff 7f 00 7e f8 33 00 88 ff ff 6d 61 58 81 ff ff ff ff 5e de 0a 81 ff ff ff ff <00> 5c e2 34 00 88 ff ff 00 00 00 00 00 00 00 00 00 00 00 00 00
[   72.696530] RIP  [<ffff880033f87d78>] 0xffff880033f87d78
[   72.696530]  RSP <ffff880035f87bc0>
[   72.696530] CR2: ffff880033f87d78
[   72.696530] ---[ end trace ad7758b9a1dccf99 ]---
[   72.696530] Kernel panic - not syncing: Fatal exception in interrupt
[   72.696530] Kernel Offset: disabled
[   72.696530] ---[ end Kernel panic - not syncing: Fatal exception in interrupt

v2: use empty initialiser instead of "{ NULL }" to avoid relying on
    first field's type.

Fixes: 38fd2af2 ("udp: Add socket based GRO and config")
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_core.c

@@ -1581,7 +1581,7 @@ int l2tp_tunnel_create(struct net *net, int fd, int version, u32 tunnel_id, u32
 	/* Mark socket as an encapsulation socket. See net/ipv4/udp.c */
 	tunnel->encap = encap;
 	if (encap == L2TP_ENCAPTYPE_UDP) {
-		struct udp_tunnel_sock_cfg udp_cfg;
+		struct udp_tunnel_sock_cfg udp_cfg = { };
 
 		udp_cfg.sk_user_data = tunnel;
 		udp_cfg.encap_type = UDP_ENCAP_L2TPINUDP;