dm thin: reinstate missing mempool_free in cell_release_singleton

Fix a significant memory leak inadvertently introduced during
simplification of cell_release_singleton() in commit
6f94a4c4 ("dm thin: fix stacked bi_next
usage").

A cell's hlist_del() must be accompanied by a mempool_free().
Use __cell_release() to do this, like before.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>