net: ipv6: ioam6: new feature tunsrc

This patch provides a new feature (i.e., "tunsrc") for the tunnel (i.e.,
"encap") mode of ioam6. Just like seg6 already does, except it is
attached to a route. The "tunsrc" is optional: when not provided (by
default), the automatic resolution is applied. Using "tunsrc" when
possible has a benefit: performance. See the comparison:
 - before (= "encap" mode): https://ibb.co/bNCzvf7
 - after (= "encap" mode with "tunsrc"): https://ibb.co/PT8L6yq



Signed-off-by: Justin Iurman <justin.iurman@uliege.be>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>