writeback: add missing requeue_io in writeback_inodes_wb

In "writeback: fix writeback_inodes_wb from writeback_inodes_sb" I
accidentally removed the requeue_io if we need to skip a superblock
because we can't pin it.  Add it back, otherwise we're getting spurious
lockups after multiple xfstests runs.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/fs-writeback.c

@@ -643,8 +643,10 @@ static void writeback_inodes_wb(struct bdi_writeback *wb,
 
 			ret = writeback_sb_inodes(sb, wb, wbc);
 		} else {
-			if (!pin_sb_for_writeback(sb))
+			if (!pin_sb_for_writeback(sb)) {
+				requeue_io(inode);
 				continue;
+			}
 			ret = writeback_sb_inodes(sb, wb, wbc);
 			drop_super(sb);
 		}