target: Indicate success if writing 0 to pi_prot_type

See https://bugzilla.redhat.com/show_bug.cgi?id=1240687

Returning 0 from a configfs store function results in infinite retries.
Reported-by: NYanko Kaneti <yaneti@declera.com>
Signed-off-by: NAndy Grover <agrover@redhat.com>
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_configfs.c

@@ -747,7 +747,7 @@ static ssize_t store_pi_prot_type(struct se_dev_attrib *da,
 	if (!dev->transport->init_prot || !dev->transport->free_prot) {
 		/* 0 is only allowed value for non-supporting backends */
 		if (flag == 0)
-			return 0;
+			return count;
 
 		pr_err("DIF protection not supported by backend: %s\n",
 		       dev->transport->name);