scsi: target: make the pi_prot_format ConfigFS path readable

pi_prot_format conversion to write-only caused userspace breakage. Make the
ConfigFS path readable again and hardcode the "0\n" content, matching
previous output.

Fixes: 6baca7601bde ("scsi: target: drop unused pi_prot_format attribute storage")
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1667505
Reported-by: Lee Duncan <[e-mail]>
Reported-by: Laura Abbott <[e-mail]>
Reviewed-by: Bart Van Assche <[e-mail]>
Signed-off-by: David Disseldorp <[e-mail]>
Signed-off-by: Martin K. Petersen <[e-mail]>

drivers/target/target_core_configfs.c

···
 	return count;
 }
 
+/* always zero, but attr needs to remain RW to avoid userspace breakage */
+static ssize_t pi_prot_format_show(struct config_item *item, char *page)
+{
+	return snprintf(page, PAGE_SIZE, "0\n");
+}
+
 static ssize_t pi_prot_format_store(struct config_item *item,
 		const char *page, size_t count)
 {
···
 CONFIGFS_ATTR(, emulate_pr);
 CONFIGFS_ATTR(, pi_prot_type);
 CONFIGFS_ATTR_RO(, hw_pi_prot_type);
-CONFIGFS_ATTR_WO(, pi_prot_format);
+CONFIGFS_ATTR(, pi_prot_format);
 CONFIGFS_ATTR(, pi_prot_verify);
 CONFIGFS_ATTR(, enforce_pr_isids);
 CONFIGFS_ATTR(, is_nonrot);