flow_dissector: Fix MPLS entropy label handling in flow dissector

Need to shift after masking to get label value for comparison.

Fixes: b3baa0fb ("mpls: Add MPLS entropy label in flow_keys")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>