ip: optimize ip4_header_checksum, take 2
ip: optimize ip4_header_checksum, take 2

- add 64-bit version
- remove byte swaps, as they are actually not needed

Type: improvement
Change-Id: Ia1efe54ee80b4ae7633b9ab36b3154c3357006d7
Signed-off-by: Damjan Marion <9141bba8b2efed526e55cf796d48631af330ad98@cisco.com>

FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp