dm ioctl: Refuse to create device named "control"

Typical userspace setups create a symlink under /dev/mapper with the
name of the device, but /dev/mapper/control is reserved for DM's control
device.  Therefore, trying to create such a device is almost certain to
be a userspace bug.
Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

drivers/md/dm-ioctl.c

@@ -767,7 +767,12 @@ static int get_target_version(struct file *filp, struct dm_ioctl *param, size_t
 static int check_name(const char *name)
 {
 	if (strchr(name, '/')) {
-		DMERR("invalid device name");
+		DMERR("device name cannot contain '/'");
+		return -EINVAL;
+	}
+
+	if (strcmp(name, DM_CONTROL_NODE) == 0) {
+		DMERR("device name cannot be \"%s\"", DM_CONTROL_NODE);
 		return -EINVAL;
 	}