target: Don't allow setting WC emulation if device doesn't support

commit 07b8dae3 upstream.

Just like for pSCSI, if the transport sets get_write_cache, then it is
not valid to enable write cache emulation for it. Return an error.

see https://bugzilla.redhat.com/show_bug.cgi?id=1082675Reviewed-by: Chris Leech <cleech@redhat.com>
Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>

drivers/target/target_core_device.c

@@ -799,10 +799,10 @@ int se_dev_set_emulate_write_cache(struct se_device *dev, int flag)
 		pr_err("emulate_write_cache not supported for pSCSI\n");
 		return -EINVAL;
 	}
-	if (dev->transport->get_write_cache) {
-		pr_warn("emulate_write_cache cannot be changed when underlying"
-			" HW reports WriteCacheEnabled, ignoring request\n");
-		return 0;
+	if (flag &&
+	    dev->transport->get_write_cache) {
+		pr_err("emulate_write_cache not supported for this device\n");
+		return -EINVAL;
 	}
 
 	dev->dev_attrib.emulate_write_cache = flag;