Btrfs - fix race between btrfs_get_sb() and umount

When mounting a btrfs file system btrfs_test_super() may attempt to
use sb->s_fs_info, the btrfs root, of a super block that is going away
and that has had the btrfs root set to NULL in its ->put_super(). But
if the super block is going away it cannot be an existing super block
so we can return false in this case.
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/super.c

@@ -566,6 +566,12 @@ static int btrfs_test_super(struct super_block *s, void *data)
 	struct btrfs_fs_devices *test_fs_devices = data;
 	struct btrfs_root *root = btrfs_sb(s);
 
+	/*
+	 * If this super block is going away, return false as it
+	 * can't match as an existing super block.
+	 */
+	if (!atomic_read(&s->s_active))
+		return 0;
 	return root->fs_info->fs_devices == test_fs_devices;
 }