MFC r346182:
When sending IPv4 packets on a SOCK_RAW socket using the IP_HDRINCL option,
ensure that the ip_hl field is valid. Furthermore, ensure that the complete
IPv4 header is contained in the first mbuf. Finally, move the length checks
before relying on them when accessing fields of the IPv4 header.

Reported by: jtl@
Reviewed by: jtl@
Differential Revision: https://reviews.freebsd.org/D19181