netdev-offload-dpdk: Fix flushing of a physdev. #2318

Job	Run time
linux gcc --disable-ssl	3m 57s
osx clang --disable-ssl	4m 22s
linux clang --disable-ssl	4m 8s
linux deb no dpdk	8m 32s
linux deb shared dpdk	13m 48s
linux gcc c99	3m 52s
linux clang c99	3m 19s
linux gcc test	12m 36s
linux clang test asan	23m 54s
linux clang test ubsan	12m 7s
linux gcc test --enable-shared	17m 51s
linux clang test --enable-shared	17m 26s
linux gcc test dpdk	25m 50s
linux clang test dpdk	21m 34s
linux gcc test -ljemalloc	11m 3s
linux clang test -ljemalloc	11m 12s
linux gcc afxdp 5.3	5m 8s
linux clang afxdp 5.3	4m 52s
linux gcc dpdk --enable-shared	18m 46s
linux clang dpdk --enable-shared	14m 31s
linux gcc dpdk-shared	15m 9s
linux clang dpdk-shared	14m 29s
linux gcc dpdk-shared --enable-shared	18m 58s
linux clang dpdk-shared --enable-shared	20m 11s
linux gcc m32 --disable-ssl	3m 29s
	5h 11m 4s

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