controller: Fix IPv6 dp flow explosion by setting flow table prefixes. #4786

Job	Run time
Prepare container	3m 18s
linux rpm fedora	6m 50s
osx clang --disable-ssl	1m 55s
linux gcc --disable-ssl	2m 43s
linux clang --disable-ssl	2m 46s
linux gcc test -300	9m 35s
linux gcc test 301-600	6m 49s
linux gcc test 601- unstable	7m 45s
linux clang test sanitizers -300	13m 1s
linux clang test sanitizers 301-600	9m 24s
linux clang test sanitizers 601- unstable	12m 17s
linux gcc test -ljemalloc -300	10m 26s
linux gcc test -ljemalloc 301-600	6m 58s
linux gcc test -ljemalloc 601- unstable	7m 32s
linux gcc system-test-dpdk dpdk -100	16m 58s
linux gcc system-test-dpdk dpdk 101- unstable	14m 57s
linux gcc system-test-userspace -100	16m 12s
linux gcc system-test-userspace 101- unstable	13m 45s
linux gcc system-test -100	18m 20s
linux gcc system-test 101- unstable	17m 58s
linux clang system-test sanitizers -100	20m 11s
linux clang system-test sanitizers 101- unstable	20m 12s
linux x86 gcc --disable-ssl	2m 37s
	4h 2m 29s

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