tun: Report "persist" flag to userspace

The TUN_PERSIST flag is not reported at all -- both TUNGETIFF, and sysfs
"flags" attribute skip one. Knowing whether a device is persistent or not
is critical for checkpoint-restore, thus I propose to add the read-only
IFF_PERSIST one for this.

Setting this new IFF_PERSIST is hardly possible, as TUNSETIFF doesn't check
for unknown flags being zero and thus there can be trash.
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -1530,6 +1530,9 @@ static int tun_flags(struct tun_struct *tun)
 	if (tun->flags & TUN_TAP_MQ)
 		flags |= IFF_MULTI_QUEUE;
 
+	if (tun->flags & TUN_PERSIST)
+		flags |= IFF_PERSIST;
+
 	return flags;
 }
 

include/uapi/linux/if_tun.h

@@ -68,6 +68,8 @@
 #define IFF_MULTI_QUEUE 0x0100
 #define IFF_ATTACH_QUEUE 0x0200
 #define IFF_DETACH_QUEUE 0x0400
+/* read-only flag */
+#define IFF_PERSIST	0x0800
 
 /* Features for GSO (TUNSETOFFLOAD). */
 #define TUN_F_CSUM	0x01	/* You can hand me unchecksummed packets. */