n_gsm: add missing __user annotations

sparse warns about incorrect types:
n_gsm.c:2638:35: warning: incorrect type in argument 1 (different address spaces)
n_gsm.c:2638:35:    expected void [noderef] <asn:1> *to
n_gsm.c:2638:35:    got void *

The ioctl handler casts its `arg' to (void *) without __user. Add that.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084949.28074-5-jslaby@suse.czSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_gsm.c

@@ -2638,11 +2638,11 @@ static int gsmld_ioctl(struct tty_struct *tty, struct file *file,
 	switch (cmd) {
 	case GSMIOC_GETCONF:
 		gsm_copy_config_values(gsm, &c);
-		if (copy_to_user((void *)arg, &c, sizeof(c)))
+		if (copy_to_user((void __user *)arg, &c, sizeof(c)))
 			return -EFAULT;
 		return 0;
 	case GSMIOC_SETCONF:
-		if (copy_from_user(&c, (void *)arg, sizeof(c)))
+		if (copy_from_user(&c, (void __user *)arg, sizeof(c)))
 			return -EFAULT;
 		return gsm_config(gsm, &c);
 	case GSMIOC_GETFIRST: