netdev-linux: Avoid deadlock in netdev_get_speed. #2069

Job	Run time
osx clang --disable-ssl	4m 2s
linux deb no dpdk	6m 31s
linux rpm fedora	5m 5s
dpdk gcc	5s
clang-analyze	18m 23s
linux gcc --disable-ssl	2m 20s
linux clang --disable-ssl	2m 50s
linux gcc c99	2m 19s
linux clang c99	2m 27s
linux gcc test	8m 3s
linux clang address,undefined test	15m 51s
linux gcc test --enable-shared	11m 9s
linux clang test --enable-shared	11m 0s
linux gcc check check-dpdk dpdk	16m 41s
linux clang check check-dpdk dpdk	17m 19s
linux gcc test -ljemalloc	8m 27s
linux clang test -ljemalloc	8m 52s
linux gcc --enable-afxdp	2m 16s
linux clang --enable-afxdp	2m 23s
linux gcc dpdk --enable-shared	3m 18s
linux clang dpdk --enable-shared	3m 20s
linux gcc dpdk-shared	2m 30s
linux clang dpdk-shared	2m 40s
linux gcc dpdk-shared --enable-shared	3m 18s
linux clang dpdk-shared --enable-shared	3m 26s
linux gcc m32 --disable-ssl	2m 32s
linux gcc check-ovsdb-cluster	6m 19s
linux gcc check-kernel -100	8m 51s
linux gcc check-kernel 100-	9m 13s
linux clang address,undefined check-kernel -100	11m 2s
linux clang address,undefined check-kernel 100-	11m 23s
linux gcc check-offloads -100	9m 23s
linux gcc check-offloads 100-	7m 15s
linux gcc dpdk check-system-userspace	10m 38s
linux clang address,undefined dpdk check-system-userspace	13m 46s
linux gcc dpdk check-system-tso	10m 25s
linux gcc dpdk check-afxdp	10m 51s
	4h 36m 13s

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