dm btree: use pop_frame in dm_btree_del to cleanup code

Remove a visited leaf straight away from the stack, rather than
marking all it's children as visited and letting it get removed on the
next iteration.  May also offer a micro optimisation in dm_btree_del.

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

drivers/md/persistent-data/dm-btree.c

@@ -287,7 +287,7 @@ int dm_btree_del(struct dm_btree_info *info, dm_block_t root)
 					info->value_type.dec(info->value_type.context,
 							     value_ptr(f->n, i));
 			}
-			f->current_child = f->nr_children;
+			pop_frame(s);
 		}
 	}