sunvnet: use icmp_ndo_send helper

Because sunvnet is calling icmp from network device context, it should use
the ndo helper so that the rate limiting applies correctly. While we're
at it, doing the additional route lookup before calling icmp_ndo_send is
superfluous, since this is the job of the icmp code in the first place.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>