RDMA/cma: Rewrite cma_req_handler() to encapsulate common code

Rewrite cma_req_handler error handling case to encapsulate
common code.

Signed-off-by: Krishna Kumar <krkumar2@in.ibm.com>
Acked-by: Sean Hefty <sean.hefty@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>