gfs2: Set qd_sync_gen in do_sync

Func do_sync was called in two places: gfs2_quota_unlock and
gfs2_quota_sync. In gfs2_quota_sync it updated qd_sync_gen to the latest
superblock sync gen, if do_sync was successful. In gfs2_quota_unlock it
didn't update the value. That can only lead to extra work, for example,
if the value is synced by gfs2_quota_unlock but still has the old value.

This patch moves the setting of qd_sync_gen inside do_sync so we are
guaranteed consistency.

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