openvswitch: Fix serialization of non-masked set actions.

Set actions consist of a regular OVS_KEY_ATTR_* attribute nested inside
of a OVS_ACTION_ATTR_SET action attribute. When converting masked actions
back to regular set actions, the inner attribute length was not changed,
ie, double the length being serialized. This patch fixes the bug.

Fixes: 83d2b9ba ("net: openvswitch: Support masked set actions.")
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>