IB/srpt: One target per port

In multipathing setups where a target system is equipped with
dual-port HCAs it is useful to have one connection per target port
instead of one connection per target HCA. Hence move the connection
list (rch_list) from struct srpt_device into struct srpt_port.

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