gtp: add GTPA_LINK info to msg sent to userspace

During a dump, this attribute is essential, it enables the userspace to
know on which interface the context is linked to.

Fixes: 459aa660 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Tested-by: Gabriel Ganne <gabriel.ganne@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>