xfs: fix bad_features2 fixups for the root filesystem

Currently the bad_features2 fixup and the alignment updates in the superblock
are skipped if we mount a filesystem read-only.  But for the root filesystem
the typical case is to mount read-only first and only later remount writeable
so we'll never perform this update at all.  It's not a big problem but means
the logs of people needing the fixup get spammed at every boot because they
never happen on disk.

Reported-by: Arkadiusz Miskiewicz <arekm@maven.pl>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>