mpls: Add MPLS entropy label in flow_keys

In flow dissector if an MPLS header contains an entropy label this is
saved in the new keyid field of flow_keys. The entropy label is
then represented in the flow hash function input.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>