flower: check unused bits in MPLS fields

Since several of the the netlink attributes used to configure the flower
classifier's MPLS TC, BOS and Label fields have additional bits which are
unused, check those bits to ensure that they are actually 0 as suggested
by Jamal.

Signed-off-by: Benjamin LaHaise <benjamin.lahaise@netronome.com>
Cc: David Miller <davem@davemloft.net>
Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Simon Horman <simon.horman@netronome.com>
Cc: Jakub Kicinski <kubakici@wp.pl>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>