gtp: Fix initialization of Flags octet in GTPv1 header

When generating a GTPv1 header in gtp1_push_header(), initialize the
'reserved' bit to zero.  All 3GPP specifications for GTPv1 from Release
99 through Release 13 agree that a transmitter shall set this bit to
zero, see e.g. Note 0 of Figure 2 in Section 6 of 3GPP TS 29.060 v13.5.0
Release 13, available from
http://www.etsi.org/deliver/etsi_ts/129000_129099/129060/13.05.00_60/ts_129060v130500p.pdf

Signed-off-by: Harald Welte <laforge@gnumonks.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gtp.c

@@ -423,11 +423,11 @@ static inline void gtp1_push_header(struct sk_buff *skb, struct pdp_ctx *pctx)
 
 	/* Bits    8  7  6  5  4  3  2	1
 	 *	  +--+--+--+--+--+--+--+--+
-	 *	  |version |PT| 1| E| S|PN|
+	 *	  |version |PT| 0| E| S|PN|
 	 *	  +--+--+--+--+--+--+--+--+
 	 *	    0  0  1  1	1  0  0  0
 	 */
-	gtp1->flags	= 0x38; /* v1, GTP-non-prime. */
+	gtp1->flags	= 0x30; /* v1, GTP-non-prime. */
 	gtp1->type	= GTP_TPDU;
 	gtp1->length	= htons(payload_len);
 	gtp1->tid	= htonl(pctx->u.v1.o_tei);