All the actual netlinks have this, and proper ICMP generation on
point-to-point links is going to need it.
The dummy "null" netlink previously didn't have this parameter; now,
it is mandatory. This is an incompatible configuration change, but
only for configurations which contain a null netlink - which we think
is only done for testing or debugging.
No other functional change.
Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>