gtp: add GTPA_LINK info to msg sent to userspace

gtp: add GTPA_LINK info to msg sent to userspace

[ Upstream commit b274e47d9e3f4dcd4ad4028a316ec22dc4533ac7 ]

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

Fixes: 459aa660eb1d ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>