The post_send() path determines if it should post directly or, schedule
the post for later.  The current logic is:

  if the swqe ring is empty or (for hfi1) wqe->length <= piothreshold
    post the send
  else
    schedule

This can allow large requests to call the send engine directly.  Large
requests can potentially produce a large number of packets prior to
returning to the caller, blocking the caller from posting more requests,
and allowing better parallel processing.

Allow the driver(s) more say in this logic (pass call_send to the driver,
rather than examining a return value).

Update hfi1/qib logic to schedule the send engine if an RC or UC message
is larger than the QP MTU size.
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>