Don't let in_proto_cksum_out() assume that the ICMP checksum field is always in the first mbuf of an mbuf chain.
Don't let in_proto_cksum_out() assume that the ICMP checksum field is
always in the first mbuf of an mbuf chain.

Thanks to henning@ and bluhm@ for their work on checksums at b2k13,
which allowed this fix to be very straightforward compared to earlier
versions.

help/feedback bluhm@ henning@
OK henning@ naddy@

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars