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: Yanko Kaneti <[e-mail]>
Signed-off-by: Andy Grover <[e-mail]>
Signed-off-by: Nicholas Bellinger <[e-mail]>

drivers/target/target_core_configfs.c

···
 	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);