parallel lookups: actual switch to rwsem

ta-da!

The main issue is the lack of down_write_killable(), so the places
like readdir.c switched to plain inode_lock(); once killable
variants of rwsem primitives appear, that'll be dealt with.

lockdep side also might need more work

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/configfs/inode.c

@@ -156,7 +156,7 @@ static void configfs_set_inode_lock_class(struct configfs_dirent *sd,
 
 	if (depth > 0) {
 		if (depth <= ARRAY_SIZE(default_group_class)) {
-			lockdep_set_class(&inode->i_mutex,
+			lockdep_set_class(&inode->i_rwsem,
 					  &default_group_class[depth - 1]);
 		} else {
 			/*