[SCSI] srp_transport: add target driver support

This adds minimum target driver support:

- srp_rport_{add,del} calls scsi_tgt_it_nexus_{create,destroy} for
target drivers.

- add a callback to notify target drivers of the nexus operation
results to srp_function_template.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Mike Christie <michaelc@cs.wisc.edu>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

include/scsi/scsi_transport_srp.h

@@ -21,7 +21,8 @@ struct srp_rport {
 };
 
 struct srp_function_template {
-	/* later */
+	/* for target drivers */
+	int (* it_nexus_response)(struct Scsi_Host *, u64, int);
 };
 
 extern struct scsi_transport_template *