openvswitch: Simplify labels length logic.

Since 23014011

 ("netfilter: conntrack: support a fixed size of 128
distinct labels"), the size of conntrack labels extension has fixed to
128 bits, so we do not need to check for labels sizes shorter than 128
at run-time.  This patch simplifies labels length logic accordingly,
but allows the conntrack labels size to be increased in the future
without breaking the build.  In the event of conntrack labels
increasing in size OVS would still be able to deal with the 128 first
label bits.

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