gfs2: Change BUG_ON to an assert_withdraw in gfs2_quota_change

Before this patch, gfs2_quota_change() would BUG_ON if the
qa_ref counter was not a positive number. This patch changes it to
be a withdraw instead. That way we can debug things more easily.

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