tr_writeid is defined at mp->m_resv structure, however, it does not
really being used when it should be..

This patch changes it to tr_writeid to fetch the correct log
reservation size.
Signed-off-by: NJie Liu <jeff.liu@oracle.com>
Signed-off-by: NDave Chinner <dchinner@redhat.com>
Reviewed-by: NMark Tinguely <tinguely@sgi.com>
Signed-off-by: NBen Myers <bpm@sgi.com>