openvswitch: Refactor ovs_nla_fill_match().

Refactor the ovs_nla_fill_match() function into separate netlink
serialization functions ovs_nla_put_{unmasked_key,mask}(). Modify
ovs_nla_put_flow() to handle attribute nesting and expose the 'is_mask'
parameter - all callers need to nest the flow, and callers have better
knowledge about whether it is serializing a mask or not.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>