xfs: keep sb_bad_features2 the same a sb_features2

Whenever we update sb_features2, we need to update sb_bad_features2
so that they remain identical on disk. This prevents future mounts
or userspace utilities from getting confused over which features the
filesystem supports.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>