dm btree remove: fix use after free in rebalance_children()

Move dm_tm_unlock() after dm_tm_dec().

Cc: stable@vger.kernel.org
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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

@@ -423,9 +423,9 @@ static int rebalance_children(struct shadow_spine *s,
 
 		memcpy(n, dm_block_data(child),
 		       dm_bm_block_size(dm_tm_get_bm(info->tm)));
-		dm_tm_unlock(info->tm, child);
 
 		dm_tm_dec(info->tm, dm_block_location(child));
+		dm_tm_unlock(info->tm, child);
 		return 0;
 	}