seg6: Extend seg6_lookup_any_nexthop() with an oif argument

seg6: Extend seg6_lookup_any_nexthop() with an oif argument

seg6_lookup_any_nexthop() is called by the different endpoint behaviors
(e.g., End, End.X) to resolve an IPv6 route. Extend the function with an
output interface argument so that it could be used to resolve a route
with a certain output interface. This will be used by subsequent patches
that will extend the End.X behavior with an output interface as an
optional argument.

ip6_route_input_lookup() cannot be used when an output interface is
specified as it ignores this parameter. Similarly, calling
ip6_pol_route() when a table ID was not specified (e.g., End.X behavior)
is wrong.

Therefore, when an output interface is specified without a table ID,
resolve the route using ip6_route_output() which will take the output
interface into account.

Note that no endpoint behavior currently passes both a table ID and an
output interface, so the oif argument passed to ip6_pol_route() is
always zero and there are no functional changes in this regard.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20250612122323.584113-2-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>