JFS: fix hang in __get_metapage

A previous patch to fix a race introduced a hang if more than one
thread is waiting for a "stale" metapage.

fs/jfs/jfs_metapage.c

@@ -239,6 +239,7 @@ struct metapage *__get_metapage(struct inode *inode, unsigned long lblock,
 		spin_unlock(&meta_lock);
 		if (test_bit(META_stale, &mp->flag)) {
 			release_metapage(mp);
+			yield();	/* Let other waiters release it, too */
 			goto again;
 		}
 		if (test_bit(META_discard, &mp->flag)) {