Properly preserve ip_tos bits for IPv4 packets

Restructure code slightly to save ip_tos bits earlier. Fix the bug
where the ip_tos field is zeroed out before assigning to the iptos
variable. Restore the ip_tos and ip_ver fields only if they have
been zeroed during the pseudo-header checksum calculation.

Reviewed by: cem, gnn, hiren
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D8053