flow_dissector: do not rely on implicit casts

This change fixes a couple of type mismatch reported by the sparse
tool, explicitly using the requested type for the offending arguments.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>