commit 7f518ad0 upstream.

The device details and mapping trees were just being decremented
before.  Now btree_del() is called to do a deep delete.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>