xfs: Introduce a new structure to hold transaction reservation items

Introduce a new structure xfs_trans_res to hold transaction
reservation item info per log ticket.

We also need to improve xfs_trans_resv_calc() by initializing the
log count as well as log flags for permanent log reservation.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>