Fix possible memory leak.

vxlan_ftable entries are sorted in descending order, due to wrong arguments
order it is possible to stop search before existing element will be found.
Then new element will be allocated in vxlan_ftable_update_locked() and can
be inserted in the list second time or trigger MPASS() assertion with
enabled INVARIANTS.

PR:		[[ https://bugs.freebsd.org/224371 | 224371 ]]