[SCSI] add srp transport class

[SCSI] add srp transport class

This adds srp transport class that works with ib_srp and ibmvscsi.

It creates only /sys/class/{srp_host,srp_remote_ports} and
srp_remote_ports has only "port_id" attribute.

viola:/sys/class/srp_remote_ports/port-0:1# ls
device  port_id  subsystem  uevent
viola:/sys/class/srp_remote_ports/port-0:1# cat port_id
4c:49:4e:55:58:20:56:49:4f:00:00:00:00:00:00:00

Signed-off-by: FUJITA Tomonori <[email protected]>
Signed-off-by: James Bottomley <[email protected]>