net: l2tp: change L2TP_ATTR_UDP_ZERO_CSUM6_{RX, TX} attribute types

The attributes L2TP_ATTR_UDP_ZERO_CSUM6_RX and
L2TP_ATTR_UDP_ZERO_CSUM6_TX are used as flags,
but is defined as a u8 in a comment.

This patch redocuments them as flags.

Adding nla_policy entries would break API, so not doing that.

CC: Tom Herbert <therbert@google.com>
Signed-off-by: NAsbjoern Sloth Toennesen <asbjorn@asbjorn.st>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/uapi/linux/l2tp.h

@@ -124,8 +124,8 @@ enum {
 	L2TP_ATTR_STATS,		/* nested */
 	L2TP_ATTR_IP6_SADDR,		/* struct in6_addr */
 	L2TP_ATTR_IP6_DADDR,		/* struct in6_addr */
-	L2TP_ATTR_UDP_ZERO_CSUM6_TX,	/* u8 */
-	L2TP_ATTR_UDP_ZERO_CSUM6_RX,	/* u8 */
+	L2TP_ATTR_UDP_ZERO_CSUM6_TX,	/* flag */
+	L2TP_ATTR_UDP_ZERO_CSUM6_RX,	/* flag */
 	L2TP_ATTR_PAD,
 	__L2TP_ATTR_MAX,
 };