Btrfs: add WARN_ONCE to detect unexpected error from merge_extent_mapping

This is a subtle case, so in order to understand the problem, it'd be good
to know the content of existing and em when any error occurs.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -562,17 +562,24 @@ int btrfs_add_extent_mapping(struct extent_map_tree *em_tree,
 			*em_in = existing;
 			ret = 0;
 		} else {
+			u64 orig_start = em->start;
+			u64 orig_len = em->len;
+
 			/*
 			 * The existing extent map is the one nearest to
 			 * the [start, start + len) range which overlaps
 			 */
 			ret = merge_extent_mapping(em_tree, existing,
 						   em, start);
-			free_extent_map(existing);
 			if (ret) {
 				free_extent_map(em);
 				*em_in = NULL;
+				WARN_ONCE(ret,
+"unexpected error %d: merge existing(start %llu len %llu) with em(start %llu len %llu)\n",
+					  ret, existing->start, existing->len,
+					  orig_start, orig_len);
 			}
+			free_extent_map(existing);
 		}
 	}