dm thin metadata: use pool locking at end of dm_pool_metadata_close

Ensure that the pool is locked during calls to __commit_transaction and
__destroy_persistent_data_objects.  Just being consistent with locking,
but reality is dm_pool_metadata_close is called once pool is being
destroyed so access to pool shouldn't be contended.

Also, use pmd_write_lock_in_core rather than __pmd_write_lock in
dm_pool_commit_metadata and rename __pmd_write_lock to
pmd_write_lock_in_core -- there was no need for the alias.

In addition, verify that the pool is locked in __commit_transaction().

Fixes: 873f258b

 ("dm thin metadata: do not write metadata if no changes occurred")
Cc: stable@vger.kernel.org
Signed-off-by: Mike Snitzer <snitzer@redhat.com>