dp-packet: Correct IPv4 checksum calculation. #3060

Job	Run time
dpdk gcc	7s
linux deb no dpdk	4m 44s
linux rpm fedora	4m 29s
osx clang --disable-ssl	1m 37s
linux gcc --disable-ssl	2m 11s
linux clang --disable-ssl	2m 6s
linux gcc c99	1m 57s
linux clang c99	2m 7s
linux gcc test	7m 45s
linux clang test asan	12m 50s
linux clang test ubsan	9m 39s
linux gcc test --enable-shared	11m 10s
linux clang test --enable-shared	10m 19s
linux gcc test dpdk	7m 45s
linux clang test dpdk	7m 55s
linux gcc test -ljemalloc	8m 14s
linux clang test -ljemalloc	8m 7s
linux gcc --enable-afxdp	2m 11s
linux clang --enable-afxdp	2m 7s
linux gcc dpdk --enable-shared	3m 33s
linux clang dpdk --enable-shared	3m 6s
linux gcc dpdk-shared	2m 26s
linux clang dpdk-shared	2m 18s
linux gcc dpdk-shared --enable-shared	2m 59s
linux clang dpdk-shared --enable-shared	2m 51s
linux gcc m32 --disable-ssl	1m 58s
	2h 6m 31s

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