flow: Add mf_is_l3_or_higher()

This is in preparation for using the same helper as part of support
for using recirculation in conjunction series of actions including
with MPLS actions that are currently not able to be translated.

In that scenario this helper will be used to test if load, move and
set_field actions require recirculation to occur.

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>