Use a separate lock to protect the orphan list, so we can stop
overloading the use of lock_super().

Port of ext4 commit 3b9d4ed2
by Theodore Ts'o <tytso@mit.edu>.

CC: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>