pfsync: fix incorrect unlock during destroy

During pfsync_clone_destroy() we clean up pending packets. This
may involve calling pfsync_undefer() or callout_drain(). We may not
hold the bucket lock during callout_drain(), but must hold it during
pfsync_undefer().

We incorrectly always released the lock, leading to assertion failures
during cleanup if there were pending deferred packets.

MFC after: 1 week
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