Merge branch 'seg6-allow-end-x-behavior-to-accept-an-oif'

Ido Schimmel says:

====================
seg6: Allow End.X behavior to accept an oif

Patches #1-#3 gradually extend the End.X behavior to accept an output
interface as an optional argument. This is needed for cases where user
space wishes to specify an IPv6 link-local address as the nexthop
address.

Patch #4 adds test cases to the existing End.X selftest to cover the new
functionality.
====================

Link: https://patch.msgid.link/20250612122323.584113-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>