IB/hfi1: Fix a latency issue for small messages

A recent performance enhancement introduced a latency issue in the
HFI message path.  The new algorithm removed a forced call send for
PIO messages and added a forced schedule event for messages larger
than the MTU.

For PIO, the schedule path can introduce thrashing that can
significantly impact the throughput for small messages.

If a message size is within the PIO threshold, always take the send
path.

Fixes: 0b79b277 ("IB/{hfi1, qib, rdmavt}: Schedule multi RC/UC packets instead of posting")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@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: Doug Ledford <dledford@redhat.com>