netdev-native-tnl: Fix inner checksums when stacking tunnels. #573

Job	Run time
linux deb no dpdk	4m 31s
linux rpm fedora	3m 54s
osx clang --disable-ssl	4m 10s
dpdk gcc	5s
clang-analyze	18m 2s
linux gcc --disable-ssl	3m 3s
linux clang --disable-ssl	2m 23s
linux gcc c99	2m 19s
linux clang c99	2m 36s
linux gcc test	7m 58s
linux clang address,undefined test	15m 59s
linux gcc test --enable-shared	10m 43s
linux clang test --enable-shared	30m 47s
linux gcc check check-dpdk dpdk	8m 12s
linux clang check check-dpdk dpdk	8m 36s
linux gcc test -ljemalloc	8m 18s
linux clang test -ljemalloc	9m 1s
linux gcc --enable-afxdp	2m 29s
linux clang --enable-afxdp	2m 29s
linux gcc dpdk --enable-shared	3m 15s
linux clang dpdk --enable-shared	3m 34s
linux gcc dpdk-shared	2m 40s
linux clang dpdk-shared	2m 39s
linux gcc dpdk-shared --enable-shared	3m 37s
linux clang dpdk-shared --enable-shared	3m 35s
linux gcc m32 --disable-ssl	2m 27s
linux gcc check-ovsdb-cluster	6m 26s
linux gcc check-kernel -100	8m 47s
linux gcc check-kernel 100-	9m 5s
linux clang address,undefined check-kernel -100	11m 5s
linux clang address,undefined check-kernel 100-	11m 59s
linux gcc check-offloads -100	9m 8s
linux gcc check-offloads 100-	7m 18s
linux gcc dpdk check-system-userspace	10m 57s
linux clang address,undefined dpdk check-system-userspace	14m 22s
linux gcc dpdk check-system-tso	10m 33s
linux gcc dpdk check-afxdp	10m 54s
	4h 37m 56s

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