IB/srpt: Rework multi-channel support

Store initiator and target port ID's once per nexus instead of in each
channel data structure. This change simplifies the duplicate connection
check in srpt_cm_req_recv().

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