gtp: rename SGSN netlink attribute

This is a mostly cosmetic rename of the SGSN netlink attribute to
the GTP link.  The justification for this is that we will be making
the module support decapsulation of "downstream" SGSN packets, in
which case the netlink parameter actually refers to the upstream GGSN
peer.  Renaming the parameter makes the relationship clearer.

The legacy name is maintained as a define in the header file in order
to not break existing code.

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>