wireguard: device: use icmp_ndo_send helper

Because wireguard is calling icmp from network device context, it should
use the ndo helper so that the rate limiting applies correctly.  This
commit adds a small test to the wireguard test suite to ensure that the
new functions continue doing the right thing in the context of
wireguard. It does this by setting up a condition that will definately
evoke an icmp error message from the driver, but along a nat'd path.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>