IB/hfi1: Remove dependence on qp->s_cur_size

The qp->s_cur_size field assumes that the S_BUSY bit protects
the field from modification after the slock is dropped. Scaling the
send engine to multiple cores would break that assumption.

Correct the issue by carrying the payload size in the txreq structure.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Don Hiatt <don.hiatt@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>