datapath: Serialize acts with original netlink len

datapath: Serialize acts with original netlink len

Previously, we used the kernel-internal netlink actions length to
calculate the size of messages to serialize back to userspace.
However,the sw_flow_actions may not be formatted exactly the same as the
actions on the wire, so store the original actions length when
de-serializing and re-use the original length when serializing.

Upstream: 8e2fed1 "openvswitch: Serialize acts with original netlink len"
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>