gfs2: invalidate_inodes() is no-op there

In fill_super() we hadn't MS_ACTIVE set yet, so there won't
be any inodes with zero i_count sitting around.

In put_super() we already have MS_ACTIVE removed *and* we
had called invalidate_inodes() since then.  So again there
won't be any inodes with zero i_count...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>