btrfs: document uuid_mutex uasge in read_chunk_tree

read_chunk_tree() calls read_one_dev(), but for seed device we have
to search the fs_uuids list, so we need the uuid_mutex. Add a comment
comment, so that we can improve this part.
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/volumes.c

@@ -6932,6 +6932,10 @@ int btrfs_read_chunk_tree(struct btrfs_fs_info *fs_info)
 	if (!path)
 		return -ENOMEM;
 
+	/*
+	 * uuid_mutex is needed only if we are mounting a sprout FS
+	 * otherwise we don't need it.
+	 */
 	mutex_lock(&uuid_mutex);
 	mutex_lock(&fs_info->chunk_mutex);