btrfs: assert extent map tree lock in add_extent_mapping

As add_extent_mapping is called from several functions, let's add the
lock annotation. The tree is going to be modified so it must be the
exclusive lock.
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent_map.c

@@ -384,6 +384,8 @@ int add_extent_mapping(struct extent_map_tree *tree,
 {
 	int ret = 0;
 
+	lockdep_assert_held_write(&tree->lock);
+
 	ret = tree_insert(&tree->map, em);
 	if (ret)
 		goto out;