vxlan: Fix uninit-value in vxlan_vnifilter_dump()

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

vxlan: Fix uninit-value in vxlan_vnifilter_dump()

KMSAN reported an uninit-value access in vxlan_vnifilter_dump() [1].

If the length of the netlink message payload is less than
sizeof(struct tunnel_msg), vxlan_vnifilter_dump() accesses bytes
beyond the message. This can lead to uninit-value access. Fix this by
returning an error in such situations.

[1]
BUG: KMSAN: uninit-value in vxlan_vnifilter_dump+0x328/0x920 drivers/net/vxlan/vxlan_vnifilter.c:422
vxlan_vnifilter_dump+0x328/0x920 drivers/net/vxlan/vxlan_vnifilter.c:422
rtnl_dumpit+0xd5/0x2f0 net/core/rtnetlink.c:6786
netlink_dump+0x93e/0x15f0 net/netlink/af_netlink.c:2317
__netlink_dump_start+0x716/0xd60 net/netlink/af_netlink.c:2432
netlink_dump_start include/linux/netlink.h:340 [inline]
rtnetlink_dump_start net/core/rtnetlink.c:6815 [inline]
rtnetlink_rcv_msg+0x1256/0x14a0 net/core/rtnetlink.c:6882
netlink_rcv_skb+0x467/0x660 net/netlink/af_netlink.c:2542
rtnetlink_rcv+0x35/0x40 net/core/rtnetlink.c:6944
netlink_unicast_kernel net/netlink/af_netlink.c:1321 [inline]
netlink_unicast+0xed6/0x1290 net/netlink/af_netlink.c:1347
netlink_sendmsg+0x1092/0x1230 net/netlink/af_netlink.c:1891
sock_sendmsg_nosec net/socket.c:711 [inline]
__sock_sendmsg+0x330/0x3d0 net/socket.c:726
____sys_sendmsg+0x7f4/0xb50 net/socket.c:2583
___sys_sendmsg+0x271/0x3b0 net/socket.c:2637
__sys_sendmsg net/socket.c:2669 [inline]
__do_sys_sendmsg net/socket.c:2674 [inline]
__se_sys_sendmsg net/socket.c:2672 [inline]
__x64_sys_sendmsg+0x211/0x3e0 net/socket.c:2672
x64_sys_call+0x3878/0x3d90 arch/x86/include/generated/asm/syscalls_64.h:47
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xd9/0x1d0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Uninit was created at:
slab_post_alloc_hook mm/slub.c:4110 [inline]
slab_alloc_node mm/slub.c:4153 [inline]
kmem_cache_alloc_node_noprof+0x800/0xe80 mm/slub.c:4205
kmalloc_reserve+0x13b/0x4b0 net/core/skbuff.c:587
__alloc_skb+0x347/0x7d0 net/core/skbuff.c:678
alloc_skb include/linux/skbuff.h:1323 [inline]
netlink_alloc_large_skb+0xa5/0x280 net/netlink/af_netlink.c:1196
netlink_sendmsg+0xac9/0x1230 net/netlink/af_netlink.c:1866
sock_sendmsg_nosec net/socket.c:711 [inline]
__sock_sendmsg+0x330/0x3d0 net/socket.c:726
____sys_sendmsg+0x7f4/0xb50 net/socket.c:2583
___sys_sendmsg+0x271/0x3b0 net/socket.c:2637
__sys_sendmsg net/socket.c:2669 [inline]
__do_sys_sendmsg net/socket.c:2674 [inline]
__se_sys_sendmsg net/socket.c:2672 [inline]
__x64_sys_sendmsg+0x211/0x3e0 net/socket.c:2672
x64_sys_call+0x3878/0x3d90 arch/x86/include/generated/asm/syscalls_64.h:47
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xd9/0x1d0 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

CPU: 0 UID: 0 PID: 30991 Comm: syz.4.10630 Not tainted 6.12.0-10694-gc44daa7e3c73 #29
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.3-3.fc41 04/01/2014