datapath: Avoid struct copy on conntrack labels.

datapath: Avoid struct copy on conntrack labels.

Older kernels have variable sized labels, and the struct itself
contains only the length, so we must memcpy the bits explicitly.

The modified system test fails on older kernels without this change.

VMware-BZ: #1841876
Fixes: 09aa98ad496d ("datapath: Inherit master's labels.")
Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>