pfsync: Destroy buckets mutexes on clone destroying interface

So that the associated data with them will be freed.

Reviewed by: kp
Fixes: 4fc65bcbe3fb pfsync: Performance improvement
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D51063

(cherry picked from commit 8213c07c20586a67bc7f7152bd7ff76c02cbc007)
(cherry picked from commit efbaf14c5b681c8f1a04a47c7bf8740134fff098)