net: vxlan: Free a leaked vetoed multicast rdst

When an rdst is rejected by a driver, the current code removes it from
the remote list, but neglects to free it. This is triggered by
tools/testing/selftests/drivers/net/mlxsw/vxlan_fdb_veto.sh and shows as
the following kmemleak trace:

unreferenced object 0xffff88817fa3d888 (size 96):
  comm "softirq", pid 0, jiffies 4372702718 (age 165.252s)
  hex dump (first 32 bytes):
    02 00 00 00 c6 33 64 03 80 f5 a2 61 81 88 ff ff  .....3d....a....
    06 df 71 ae ff ff ff ff 0c 00 00 00 04 d2 6a 6b  ..q...........jk
  backtrace:
    [<00000000296b27ac>] kmem_cache_alloc_trace+0x1ae/0x370
    [<0000000075c86dc6>] vxlan_fdb_append.part.12+0x62/0x3b0 [vxlan]
    [<00000000e0414b63>] vxlan_fdb_update+0xc61/0x1020 [vxlan]
    [<00000000f330c4bd>] vxlan_fdb_add+0x2e8/0x3d0 [vxlan]
    [<0000000008f81c2c>] rtnl_fdb_add+0x4c2/0xa10
    [<00000000bdc4b270>] rtnetlink_rcv_msg+0x6dd/0x970
    [<000000006701f2ce>] netlink_rcv_skb+0x290/0x410
    [<00000000c08a5487>] rtnetlink_rcv+0x15/0x20
    [<00000000d5f54b1e>] netlink_unicast+0x43f/0x5e0
    [<00000000db4336bb>] netlink_sendmsg+0x789/0xcd0
    [<00000000e1ee26b6>] sock_sendmsg+0xba/0x100
    [<00000000ba409802>] ___sys_sendmsg+0x631/0x960
    [<000000003c332113>] __sys_sendmsg+0xea/0x180
    [<00000000f4139144>] __x64_sys_sendmsg+0x78/0xb0
    [<000000006d1ddc59>] do_syscall_64+0x94/0x410
    [<00000000c8defa9a>] entry_SYSCALL_64_after_hwframe+0x49/0xbe

Move vxlan_dst_free() up and schedule a call thereof to plug this leak.

Fixes: 61f46fe8 ("vxlan: Allow vetoing of FDB notifications")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>