gtp: use icmp_ndo_send helper

Because gtp is calling icmp from network device context, it should use
the ndo helper so that the rate limiting applies correctly.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Harald Welte <laforge@gnumonks.org>
Signed-off-by: David S. Miller <davem@davemloft.net>