target: Allow userspace to write 1 to attrib/emulate_fua_write

Before 4.0, reading attrib/emulate_fua_write has returned 1. Saved
configs created on a pre-4.0 kernel will try to write that back when
restoring LIO configuration. This should succeed with no effect,
and issue a warning.

See https://bugzilla.redhat.com/show_bug.cgi?id=1206184Reported-by: NYanko Kaneti <yaneti@declera.com>
Reported-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NAndy Grover <agrover@redhat.com>
Signed-off-by: NNicholas Bellinger <nab@linux-iscsi.org>

drivers/target/target_core_device.c

@@ -781,8 +781,8 @@ int se_dev_set_emulate_fua_write(struct se_device *dev, int flag)
 	}
 	if (flag &&
 	    dev->transport->get_write_cache) {
-		pr_err("emulate_fua_write not supported for this device\n");
-		return -EINVAL;
+		pr_warn("emulate_fua_write not supported for this device, ignoring\n");
+		return 0;
 	}
 	if (dev->export_count) {
 		pr_err("emulate_fua_write cannot be changed with active"