vxlan_vnifilter_dump_dev() assumes it is called only
for vxlan devices. Make sure it is the case.

BUG: KASAN: slab-out-of-bounds in vxlan_vnifilter_dump_dev+0x9a0/0xb40 drivers/net/vxlan/vxlan_vnifilter.c:349
Read of size 4 at addr ffff888060d1ce70 by task syz-executor.3/17662

CPU: 0 PID: 17662 Comm: syz-executor.3 Tainted: G        W         5.17.0-syzkaller-12888-g77c9387c #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
 print_address_description.constprop.0.cold+0xeb/0x495 mm/kasan/report.c:313
 print_report mm/kasan/report.c:429 [inline]
 kasan_report.cold+0xf4/0x1c6 mm/kasan/report.c:491
 vxlan_vnifilter_dump_dev+0x9a0/0xb40 drivers/net/vxlan/vxlan_vnifilter.c:349
 vxlan_vnifilter_dump+0x3ff/0x650 drivers/net/vxlan/vxlan_vnifilter.c:428
 netlink_dump+0x4b5/0xb70 net/netlink/af_netlink.c:2270
 __netlink_dump_start+0x647/0x900 net/netlink/af_netlink.c:2375
 netlink_dump_start include/linux/netlink.h:245 [inline]
 rtnetlink_rcv_msg+0x70c/0xb80 net/core/rtnetlink.c:5953
 netlink_rcv_skb+0x153/0x420 net/netlink/af_netlink.c:2496
 netlink_unicast_kernel net/netlink/af_netlink.c:1319 [inline]
 netlink_unicast+0x543/0x7f0 net/netlink/af_netlink.c:1345
 netlink_sendmsg+0x904/0xe00 net/netlink/af_netlink.c:1921
 sock_sendmsg_nosec net/socket.c:705 [inline]
 sock_sendmsg+0xcf/0x120 net/socket.c:725
 ____sys_sendmsg+0x6e2/0x800 net/socket.c:2413
 ___sys_sendmsg+0xf3/0x170 net/socket.c:2467
 __sys_sendmsg+0xe5/0x1b0 net/socket.c:2496
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x35/0x80 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f87b8e89049

Fixes: f9c4bb0b ("vxlan: vni filtering support on collect metadata device")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Roopa Prabhu <roopa@nvidia.com>
Link: https://lore.kernel.org/r/20220330194643.2706132-1-eric.dumazet@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>