vxlan: check vxlan_vnigroup_init() return value

In the Linux kernel, the following vulnerability has been resolved:

vxlan: check vxlan_vnigroup_init() return value

vxlan_init() must check vxlan_vnigroup_init() success
otherwise a crash happens later, spotted by syzbot.

Oops: general protection fault, probably for non-canonical address 0xdffffc000000002c: 0000 [#1] PREEMPT SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000160-0x0000000000000167]
CPU: 0 UID: 0 PID: 7313 Comm: syz-executor147 Not tainted 6.14.0-rc1-syzkaller-00276-g69b54314c975 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:vxlan_vnigroup_uninit+0x89/0x500 drivers/net/vxlan/vxlan_vnifilter.c:912
Code: 00 48 8b 44 24 08 4c 8b b0 98 41 00 00 49 8d 86 60 01 00 00 48 89 c2 48 89 44 24 10 48 b8 00 00 00 00 00 fc ff df 48 c1 ea 03 <80> 3c 02 00 0f 85 4d 04 00 00 49 8b 86 60 01 00 00 48 ba 00 00 00
RSP: 0018:ffffc9000cc1eea8 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000001 RCX: ffffffff8672effb
RDX: 000000000000002c RSI: ffffffff8672ecb9 RDI: ffff8880461b4f18
RBP: ffff8880461b4ef4 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000020000
R13: ffff8880461b0d80 R14: 0000000000000000 R15: dffffc0000000000
FS: 00007fecfa95d6c0(0000) GS:ffff88806a600000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fecfa95cfb8 CR3: 000000004472c000 CR4: 0000000000352ef0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
vxlan_uninit+0x1ab/0x200 drivers/net/vxlan/vxlan_core.c:2942
unregister_netdevice_many_notify+0x12d6/0x1f30 net/core/dev.c:11824
unregister_netdevice_many net/core/dev.c:11866 [inline]
unregister_netdevice_queue+0x307/0x3f0 net/core/dev.c:11736
register_netdevice+0x1829/0x1eb0 net/core/dev.c:10901
__vxlan_dev_create+0x7c6/0xa30 drivers/net/vxlan/vxlan_core.c:3981
vxlan_newlink+0xd1/0x130 drivers/net/vxlan/vxlan_core.c:4407
rtnl_newlink_create net/core/rtnetlink.c:3795 [inline]
__rtnl_newlink net/core/rtnetlink.c:3906 [inline]