I'm now cleaning up all users of dst_pmtu with the aim of replacing
dst_pmtu with dst_mtu.  I'm going to start with the ones that actually
fix bugs.

This patch fixes the length calculation in icmp_send.  As it is
we're overestimating the space available by including the space
that would be used up by IPsec encapsulation.

IPv6 doesn't have this problem since its calculation is based
on 1280 instead of the PMTU.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>