vxlan: vnifilter: Fix unlocked deletion of default FDB entry
In the Linux kernel, the following vulnerability has been resolved:

vxlan: vnifilter: Fix unlocked deletion of default FDB entry

When a VNI is deleted from a VXLAN device in 'vnifilter' mode, the FDB
entry associated with the default remote (assuming one was configured)
is deleted without holding the hash lock. This is wrong and will result
in a warning [1] being generated by the lockdep annotation that was
added by commit ebe642067455 ("vxlan: Create wrappers for FDB lookup").

Reproducer:

 # ip link add vx0 up type vxlan dstport 4789 external vnifilter local 192.0.2.1
 # bridge vni add vni 10010 remote 198.51.100.1 dev vx0
 # bridge vni del vni 10010 dev vx0

Fix by acquiring the hash lock before the deletion and releasing it
afterwards. Blame the original commit that introduced the issue rather
than the one that exposed it.

[1]
WARNING: CPU: 3 PID: 392 at drivers/net/vxlan/vxlan_core.c:417 vxlan_find_mac+0x17f/0x1a0
[...]
RIP: 0010:vxlan_find_mac+0x17f/0x1a0
[...]
Call Trace:
 <TASK>
 __vxlan_fdb_delete+0xbe/0x560
 vxlan_vni_delete_group+0x2ba/0x940
 vxlan_vni_del.isra.0+0x15f/0x580
 vxlan_process_vni_filter+0x38b/0x7b0
 vxlan_vnifilter_process+0x3bb/0x510
 rtnetlink_rcv_msg+0x2f7/0xb70
 netlink_rcv_skb+0x131/0x360
 netlink_unicast+0x426/0x710
 netlink_sendmsg+0x75a/0xc20
 __sock_sendmsg+0xc1/0x150
 ____sys_sendmsg+0x5aa/0x7b0
 ___sys_sendmsg+0xfc/0x180
 __sys_sendmsg+0x121/0x1b0
 do_syscall_64+0xbb/0x1d0
 entry_SYSCALL_64_after_hwframe+0x4b/0x53