Btrfs: do not ignore errors from btrfs_cleanup_fs_roots() when mounting

There used to be a BUG_ON(ret) there before EH patch (79787eaa

) went in.
Bail out with EINVAL.

Cc: David Sterba <dsterba@suse.cz>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>