tty: n_gsm: mark unusable ioctl structure fields accordingly

gsm_config and gsm_netconfig includes unused fields that have been included
to allow future extension without changing the structure size.
Unfortunately, no checks have been included for these field. The actual
value set by old user space code remains undefined.
This means that future extensions can not use these fields without breaking
old user space code which may set unexpected values.

Mark these fields accordingly to avoid breaking code changes.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>
Link: https://lore.kernel.org/r/20230206114606.2133-1-daniel.starke@siemens.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/uapi/linux/gsmmux.h

@@ -19,8 +19,7 @@ struct gsm_config
 	unsigned int mtu;
 	unsigned int k;
 	unsigned int i;
-	unsigned int unused[8];		/* Padding for expansion without
-					   breaking stuff */
+	unsigned int unused[8];	/* Can not be used */
 };
 
 #define GSMIOC_GETCONF		_IOR('G', 0, struct gsm_config)
@@ -29,9 +28,9 @@ struct gsm_config
 struct gsm_netconfig {
 	unsigned int adaption;  /* Adaption to use in network mode */
 	unsigned short protocol;/* Protocol to use - only ETH_P_IP supported */
-	unsigned short unused2;
+	unsigned short unused2;	/* Can not be used */
 	char if_name[IFNAMSIZ];	/* interface name format string */
-	__u8 unused[28];        /* For future use */
+	__u8 unused[28];        /* Can not be used */
 };
 
 #define GSMIOC_ENABLE_NET      _IOW('G', 2, struct gsm_netconfig)