IB/srp: Refactor srp_send_req()

This patch does not change any functionality but prepares for the patch
that adds RDMA_CM support by making the RDMA_CM patch much easier to
read.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>