seg6: add per-route tunnel source address

seg6: add per-route tunnel source address

Add SEG6_IPTUNNEL_SRC in the uapi for users to configure a specific
tunnel source address. Make seg6_iptunnel handle the new attribute
correctly. It has priority over the configured per-netns tunnel
source address, if any.

Cc: David Ahern <dsahern@kernel.org>
Signed-off-by: Justin Iurman <justin.iurman@6wind.com>
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Link: https://patch.msgid.link/20260324091434.359341-2-justin.iurman@6wind.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>