Since Linux kernel 4.2, net.ipv6.auto_flowlabels is set by default, and
therefore the request and reply may have different IPv6 flow label.
Bug: 31444338
Test: On a kernel 4.4 board, run com.android.cts.net.HostsideNetworkTests#testVpn
Test: On a kernel 3.18 board, run echo 1 > /proc/sys/net/ipv6/auto_flowlabels, then
com.android.cts.net.HostsideNetworkTests#testVpn
Change-Id: I913bbf91574239a24cb32ae908834eb951ea2010