gtp: support SGSN-side tunnels

The GTP-tunnel driver is explicitly GGSN-side as it searches for PDP
contexts based on the incoming packets _destination_ address.  If we
want to place ourselves on the SGSN side of the  tunnel, then we want
to be identifying PDP contexts based on _source_ address.

Let it be noted that in a "real" configuration this module would never
be used:  the SGSN normally does not see IP packets as input.  The
justification for this functionality is for PGW load-testing applications
where the input to the SGSN is locally generally IP traffic.

This patch adds a "role" argument at GTP-link creation time to specify
whether we are on the GGSN or SGSN side of the tunnel; this flag is then
used to determine which part of the IP packet to use in determining
the PDP context.

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