selftests: add IPv4 unicast extensions tests

Add selftests for kernel behavior with regard to various classes of
unallocated/reserved IPv4 addresses, checking whether or not these
addresses can be assigned as unicast addresses on links and used in
routing.

Expect the current kernel behavior at the time of this patch. That is:

* 0/8 and 240/4 may be used as unicast, with the exceptions of 0.0.0.0
  and 255.255.255.255;
* the lowest address in a subnet may only be used as a broadcast address;
* 127/8 may not be used as unicast (the route_localnet option, which is
  disabled by default, still leaves it treated slightly specially);
* 224/4 may not be used as unicast.

Signed-off-by: Seth David Schoen <schoen@loyalty.org>
Suggested-by: John Gilmore <gnu@toad.com>
Acked-by: Dave Taht <dave.taht@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20210126040834.GR24989@frotz.zork.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>