Merge branch 'add-support-for-per-route-seg6-tunsrc'

Justin Iurman says:

====================
Add support for per-route seg6 tunsrc

This series adds support for the new per-route seg6 "tunsrc" parameter.
Selftests are extended to make sure it works as expected.

Example with the iproute2-next companion patch:

ip -6 r a 2001:db8:1::/64 encap seg6 mode encap tunsrc 2001:db8:ab::
    segs 2001:db8:42::1,2001:db8:ffff::2 dev eth0
====================

Link: https://patch.msgid.link/20260324091434.359341-1-justin.iurman@6wind.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>