staging/rdma/hfi1: fix panic in send engine

staging/rdma/hfi1: fix panic in send engine

The send engine wasn't correctly handling
pre-built packets, and worse, the pointer to
a packet state's txreq wasn't initialized correctly.

To fix:
- all waiters need to save any prebuilt packets
  (smda waits already did)
- the progress routine needs to handle a QPs prebuilt packet
  and initialize the txreq pointer properly

To keep SDMA working, the dma send code needs to see if
a packet has been built already. If not the code will build
it.

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