openvswitch: Rename LABEL->LABELS

Conntrack LABELS (plural) are exposed by conntrack; rename the OVS name
for these to be consistent with conntrack.

Fixes: c2ac6673 "openvswitch: Allow matching on conntrack label"
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>