btrfs: use btrfs_warn() to log message at btrfs_add_extent_mapping()

At btrfs_add_extent_mapping(), if we failed to merge the extent map, which
is unexpected and theoretically should never happen, we use WARN_ONCE() to
log a message which is not great because we don't get information about
which filesystem it relates to in case we have multiple btrfs filesystems
mounted. So change this to use btrfs_warn() and surround the error check
with WARN_ON() so we always get a useful stack trace and the condition is
flagged as "unlikely" since it's not expected to ever happen.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/extent_map.c

@@ -629,13 +629,13 @@ int btrfs_add_extent_mapping(struct btrfs_fs_info *fs_info,
 			 */
 			ret = merge_extent_mapping(em_tree, existing,
 						   em, start);
-			if (ret) {
+			if (WARN_ON(ret)) {
 				free_extent_map(em);
 				*em_in = NULL;
-				WARN_ONCE(ret,
-"extent map merge error existing [%llu, %llu) with em [%llu, %llu) start %llu\n",
-					  existing->start, extent_map_end(existing),
-					  orig_start, orig_start + orig_len, start);
+				btrfs_warn(fs_info,
+"extent map merge error existing [%llu, %llu) with em [%llu, %llu) start %llu",
+					   existing->start, extent_map_end(existing),
+					   orig_start, orig_start + orig_len, start);
 			}
 			free_extent_map(existing);
 		}