l2tp: add comment in API header that L2TP_ATTR_OFFSET is not used

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/l2tp.h

@@ -94,7 +94,7 @@ enum {
 	L2TP_ATTR_NONE,			/* no data */
 	L2TP_ATTR_PW_TYPE,		/* u16, enum l2tp_pwtype */
 	L2TP_ATTR_ENCAP_TYPE,		/* u16, enum l2tp_encap_type */
-	L2TP_ATTR_OFFSET,		/* u16 */
+	L2TP_ATTR_OFFSET,		/* u16 (not used) */
 	L2TP_ATTR_DATA_SEQ,		/* u16 */
 	L2TP_ATTR_L2SPEC_TYPE,		/* u8, enum l2tp_l2spec_type */
 	L2TP_ATTR_L2SPEC_LEN,		/* u8, enum l2tp_l2spec_type */