openvswitch: Unionize ovs_key_ct_label with a u32 array.

Make the array of labels in struct ovs_key_ct_label an union, adding a
u32 array of the same byte size as the existing u8 array.  It is
faster to loop through the labels 32 bits at the time, which is also
the alignment of netlink attributes.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@ovn.org>
Signed-off-by: David S. Miller <davem@davemloft.net>