target: Allow non-supporting backends to set pi_prot_type to 0

Userspace tools assume if a value is read from configfs, it is valid
and will not cause an error if the same value is written back. The only
valid value for pi_prot_type for backends not supporting DIF is 0, so allow
this particular value to be set without returning an error.
Reported-by: Krzysztof Chojnowski <frirajder@gmail.com>
Signed-off-by: Andy Grover <agrover@redhat.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Cc: stable@vger.kernel.org # 3.14+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_device.c

@@ -936,6 +936,10 @@ int se_dev_set_pi_prot_type(struct se_device *dev, int flag)
 		return 0;
 	}
 	if (!dev->transport->init_prot || !dev->transport->free_prot) {
+		/* 0 is only allowed value for non-supporting backends */
+		if (flag == 0)
+			return 0;
+
 		pr_err("DIF protection not supported by backend: %s\n",
 		       dev->transport->name);
 		return -ENOSYS;