rdma/cxgb4: Add support for kernel mode SRQ's

This patch implements the srq specific verbs such as create/destroy/modify
and post_srq_recv. And adds srq specific structures and defines to t4.h
and uapi.

Also updates the cq poll logic to deal with completions that are
associated with the SRQ's.

This patch also handles kernel mode SRQ_LIMIT events as well as flushed
SRQ buffers

Signed-off-by: Raju Rangoju <rajur@chelsio.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>