netdev-dpdk: Fix race condition in mempool information dump. #3030

Job	Run time
dpdk gcc	8s
linux rpm fedora	4m 9s
osx clang --disable-ssl	1m 44s
linux deb no dpdk	6m 11s
clang-analyze	9m 35s
linux gcc --disable-ssl	3m 44s
linux clang --disable-ssl	2m 30s
linux gcc c99	2m 39s
linux clang c99	3m 0s
linux gcc test	8m 33s
linux clang address,undefined test	16m 50s
linux gcc test --enable-shared	11m 57s
linux clang test --enable-shared	11m 34s
linux gcc check check-dpdk dpdk	18m 6s
linux clang check check-dpdk dpdk	18m 41s
linux gcc test -ljemalloc	8m 55s
linux clang test -ljemalloc	10m 5s
linux gcc --enable-afxdp	2m 59s
linux clang --enable-afxdp	2m 42s
linux gcc dpdk --enable-shared	3m 45s
linux clang dpdk --enable-shared	3m 30s
linux gcc dpdk-shared	2m 39s
linux clang dpdk-shared	3m 32s
linux gcc dpdk-shared --enable-shared	3m 27s
linux clang dpdk-shared --enable-shared	3m 46s
linux gcc m32 --disable-ssl	2m 40s
linux gcc check-ovsdb-cluster	7m 10s
linux gcc check-kernel -100	10m 58s
linux gcc check-kernel 100-	10m 17s
linux clang address,undefined check-kernel -100	11m 33s
linux clang address,undefined check-kernel 100-	12m 2s
linux gcc check-offloads -100	9m 24s
linux gcc check-offloads 100-	8m 25s
linux gcc dpdk check-system-userspace	11m 6s
linux clang address,undefined dpdk check-system-userspace	14m 43s
linux gcc dpdk check-system-tso	12m 1s
linux gcc dpdk check-afxdp	11m 34s
	4h 46m 34s

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