Some hardware drivers requires additional checks on send WRs.  Create an
optional call back to allow hardware drivers to reject a send WR.
Reviewed-by: NDennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: NIra Weiny <ira.weiny@intel.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>