staging/rdma/hfi1: add common routine for queuing acks

This patch is a prelimary patch required to
coalesce acks.

The routine to "schedule" a QP for sending a NAK is
now centralized in rc_defer_ack().  The flag is changed
for clarity since the all acks will potentially use
the deferral  mechanism.

Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>