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>