seg6: Call seg6_lookup_any_nexthop() from End.X behavior

seg6: Call seg6_lookup_any_nexthop() from End.X behavior

seg6_lookup_nexthop() is a wrapper around seg6_lookup_any_nexthop().
Change End.X behavior to invoke seg6_lookup_any_nexthop() directly so
that we would not need to expose the new output interface argument
outside of the seg6local module.

No functional changes intended.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20250612122323.584113-3-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>