meta-flow: Fix nw_frag mask while parsing from string. #3152

Job	Run time
osx clang --disable-ssl	1m 32s
linux deb no dpdk	6m 59s
linux deb shared dpdk	7m 48s
linux gcc --disable-ssl	2m 6s
linux clang --disable-ssl	2m 11s
linux gcc c99	2m 17s
linux clang c99	2m 33s
linux gcc test	7m 20s
linux clang test asan	12m 32s
linux clang test ubsan	8m 43s
linux gcc test --enable-shared	10m 40s
linux clang test --enable-shared	11m 13s
linux gcc test dpdk	8m 19s
linux clang test dpdk	8m 28s
linux gcc test -ljemalloc	7m 38s
linux clang test -ljemalloc	7m 53s
linux gcc afxdp 5.4	2m 57s
linux clang afxdp 5.4	3m 22s
linux gcc dpdk --enable-shared	3m 18s
linux clang dpdk --enable-shared	3m 11s
linux gcc dpdk-shared	2m 33s
linux clang dpdk-shared	2m 48s
linux gcc dpdk-shared --enable-shared	3m 1s
linux clang dpdk-shared --enable-shared	3m 0s
linux gcc m32 --disable-ssl	2m 12s
	2h 14m 34s

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