vxlan: Fix kernel crash on rmmod.

vxlan: Fix kernel crash on rmmod.

vxlan exit module unregisters vxlan net and then it unregisters
rtnl ops which triggers vxlan_dellink() from __rtnl_kill_links().
vxlan_dellink() deletes vxlan-dev from vxlan_list which has
list-head in vxlan-net-struct but that is already gone due to
net-unregister. That is how we are getting following crash.

Following commit fixes the crash by fixing module exit path.

BUG: unable to handle kernel paging request at ffff8804102c8000
IP: [<ffffffff812cc5e9>] __list_del_entry+0x29/0xd0
PGD 2972067 PUD 83e019067 PMD 83df97067 PTE 80000004102c8060
Oops: 0000 [#1] SMP DEBUG_PAGEALLOC
Modules linked in: ---
CPU: 19 PID: 6712 Comm: rmmod Tainted: GF            3.10.0+ #95
Hardware name: Dell Inc. PowerEdge R620/0KCKR5, BIOS 1.4.8 10/25/2012
task: ffff88080c47c580 ti: ffff88080ac50000 task.ti: ffff88080ac50000
RIP: 0010:[<ffffffff812cc5e9>]  [<ffffffff812cc5e9>]
__list_del_entry+0x29/0xd0
RSP: 0018:ffff88080ac51e08  EFLAGS: 00010206
RAX: ffff8804102c8000 RBX: ffff88040f0d4b10 RCX: dead000000200200
RDX: ffff8804102c8000 RSI: ffff88080ac51e58 RDI: ffff88040f0d4b10
RBP: ffff88080ac51e08 R08: 0000000000000001 R09: 2222222222222222
R10: 2222222222222222 R11: 2222222222222222 R12: ffff88080ac51e58
R13: ffffffffa07b8840 R14: ffffffff81ae48c0 R15: ffff88080ac51e58
FS:  00007f9ef105c700(0000) GS:ffff88082a800000(0000)
knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffff8804102c8000 CR3: 00000008227e5000 CR4: 00000000000407e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
Stack:
 ffff88080ac51e28 ffffffff812cc6a1 2222222222222222 ffff88040f0d4000
 ffff88080ac51e48 ffffffffa07b3311 ffff88040f0d4000 ffffffff81ae49c8
 ffff88080ac51e98 ffffffff81492fc2 ffff88080ac51e58 ffff88080ac51e58
Call Trace:
 [<ffffffff812cc6a1>] list_del+0x11/0x40
 [<ffffffffa07b3311>] vxlan_dellink+0x51/0x70 [vxlan]
 [<ffffffff81492fc2>] __rtnl_link_unregister+0xa2/0xb0
 [<ffffffff8149448e>] rtnl_link_unregister+0x1e/0x30
 [<ffffffffa07b7b7c>] vxlan_cleanup_module+0x1c/0x2f [vxlan]
 [<ffffffff810c9b31>] SyS_delete_module+0x1d1/0x2c0
 [<ffffffff812b8a0e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
 [<ffffffff81582f42>] system_call_fastpath+0x16/0x1b
Code: eb 9f 55 48 8b 17 48 b9 00 01 10 00 00 00 ad de 48 8b 47 08 48 89
e5 48 39 ca 74 29 48 b9 00 02 20 00 00 00 ad de 48 39 c8 74 7a <4c> 8b
00 4c 39 c7 75 53 4c 8b 42 08 4c 39 c7 75 2b 48 89 42 08
RIP  [<ffffffff812cc5e9>] __list_del_entry+0x29/0xd0
 RSP <ffff88080ac51e08>
CR2: ffff8804102c8000

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>