seg6: Allow End.X behavior to accept an oif

seg6: Allow End.X behavior to accept an oif

Extend the End.X behavior to accept an output interface as an optional
attribute and make use of it when resolving a route. This is needed when
user space wants to use a link-local address as the nexthop address.

Before:

 # ip route add 2001:db8:1::/64 encap seg6local action End.X nh6 fe80::1 oif eth0 dev sr6
 # ip route add 2001:db8:2::/64 encap seg6local action End.X nh6 2001:db8:10::1 dev sr6
 $ ip -6 route show
 2001:db8:1::/64  encap seg6local action End.X nh6 fe80::1 dev sr6 metric 1024 pref medium
 2001:db8:2::/64  encap seg6local action End.X nh6 2001:db8:10::1 dev sr6 metric 1024 pref medium

After:

 # ip route add 2001:db8:1::/64 encap seg6local action End.X nh6 fe80::1 oif eth0 dev sr6
 # ip route add 2001:db8:2::/64 encap seg6local action End.X nh6 2001:db8:10::1 dev sr6
 $ ip -6 route show
 2001:db8:1::/64  encap seg6local action End.X nh6 fe80::1 oif eth0 dev sr6 metric 1024 pref medium
 2001:db8:2::/64  encap seg6local action End.X nh6 2001:db8:10::1 dev sr6 metric 1024 pref medium

Note that the oif attribute is not dumped to user space when it was not
specified (as an oif of 0) since each entry keeps track of the optional
attributes that it parsed during configuration (see struct
seg6_local_lwt::parsed_optattrs).

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20250612122323.584113-4-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>