writeback: fix pin_sb_for_writeback

We need to check for s_instances to make sure we don't bother working
against a filesystem that is beeing unmounted, and we need to call
put_super to make sure a superblock is freed when we race against
umount.  Also no need to keep sb_lock after we got a reference on it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>

fs/fs-writeback.c

@@ -534,19 +534,21 @@ writeback_single_inode(struct inode *inode, struct writeback_control *wbc)
 static bool pin_sb_for_writeback(struct super_block *sb)
 {
 	spin_lock(&sb_lock);
+	if (list_empty(&sb->s_instances)) {
+		spin_unlock(&sb_lock);
+		return false;
+	}
+
 	sb->s_count++;
+	spin_unlock(&sb_lock);
+
 	if (down_read_trylock(&sb->s_umount)) {
-		if (sb->s_root) {
-			spin_unlock(&sb_lock);
+		if (sb->s_root)
 			return true;
-		}
-		/*
-		 * umounted, drop rwsem again and fall through to failure
-		 */
 		up_read(&sb->s_umount);
 	}
-	sb->s_count--;
-	spin_unlock(&sb_lock);
+
+	put_super(sb);
 	return false;
 }