gfs2: Be more careful with the quota sync generation

The quota sync generation is only ever updated under sd_quota_sync_mutex
by gfs2_quota_sync(), but its current value is fetched ouside of that
mutex, so use WRITE_ONCE() and READ_ONCE() when accessing it without
holding that mutex.

Pass the current sync generation to do_sync() from its callers to ensure
that we're not recording the wrong generation when the syncing is
done.  Also, make sure that qd->qd_sync_gen only ever moves forward.

In gfs2_quota_sync(), only write the new sync generation when we know
that there are changes.  This eliminates the need for function
sd_changed(), which we will remove in the next commit.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>