staging/rdma/hfi1: optionally prescan rx queue for {B, F}ECNs - UC, RC

To more rapidly respond to Explicit Congestion Notifications, prescan the
receive queue, and process FECNs, and BECNs first.  When a UC, or RC packet
containing a FECN, or BECN is found, immediately react to the ECN (either by
returning a CNP, or adjusting the injection rate). Afterward, the packet will
be processed normally.

Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Arthur Kepner <arthur.kepner@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>