The BUG_ON() at the start of scoutfs_lock_destroy() was intended to
ensure that scoutfs_lock_shutdown() had been called first. But that
doesn't happen in the case where we get an error during mount.
The _destroy() function is careful to notice active use and only tears
down resources that were created. The BUG_ON() can just be removed.
Signed-off-by: Zach Brown <zab@versity.com>