Ensure that the buffer length and the length provided in the IPv4
header match when using a raw socket to send IPv4 packets and
providing the header. If they don't match, let send return -1
and set errno to EINVAL.

Before this patch is was only enforced that the length in the header
is not larger then the buffer length.

PR: 212283
Reviewed by: ae, gnn
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D9161