IB/hfi1: Remove usage of qp->s_cur_sge

The s_cur_sge field in the qp structure holds a pointer to the
SGE of the currently processed WQE. It assumes the protection
of the RVT_S_BUSY flag to prevent the changing of this field
while the send engine is using it. This scheme works as long
as there is only one instance of the send engine running at a
time.

Scaling of the send engine to multiple cores would break this
assumption as there could be multiple instances of the send engine
running on different CPUs. This opens a window where the QP's
RVT_S_BUSY flag is not set but the send engine is still running.

To prevent accidental changing of the s_cur_sge pointer, the QP's
dependence on it is removed. The SGE pointer is now stored in the
verbs_txreq, which is a per-packet data structure. This ensures
that each individual packet has it's own pointer, which is setup
while the RVT_S_BUSY flag is set.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mitko Haralanov <mitko.haralanov@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>