IB/hfi1: Unreserve a reserved request when it is completed

Currently, When a reserved operation is completed, its entry in the send
queue will not be unreserved, which leads to the miscalculation of
qp->s_avail and thus the triggering of a WARN_ON call trace. This patch
fixes the problem by unreserving the reserved operation when it is
completed.

Fixes: 856cc4c2 ("IB/hfi1: Add the capability for reserved operations")
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Kaike Wan <kaike.wan@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>