net/hsr: add interlink to fill_info output

net/hsr: add interlink to fill_info output

Currently, it is possible to configure the interlink
port, but no way to read it back from userspace.

Add it to the output of hsr_fill_info(), so it can be
read from userspace, for example:

$ ip -d link show hsr0
12: hsr0: <BROADCAST,MULTICAST> mtu ...
...
hsr slave1 veth0 slave2 veth1 interlink veth2 ...

Signed-off-by: Jan Vaclav <jvaclav@redhat.com>
Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
Link: https://patch.msgid.link/20251015101001.25670-2-jvaclav@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>