xfs: make use of xfs_calc_buf_res() in xfs_trans.c

Refining the existing reservations with xfs_calc_buf_res() in xfs_trans.c
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
CC: Dave Chinner <david@fromorbit.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>