IB/srp: Make CM timeout dependent on subnet timeout

For small networks it is safe to reduce the subnet timeout from
its default value (18 for opensm) to 16. Make the SRP CM timeout
dependent on the subnet timeout such that decreasing the subnet
timeout also causes SRP failover and failback to occur faster.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Doug Ledford <dledford@redhat.com>