lib/match: Do not format undefined fields.

lib/match: Do not format undefined fields.

Add function flow_wildcards_init_for_packet() that can be used to set
sensible wildcards when megaflows are disabled.  Before this, we set
all the mask bits to ones, which caused printing tunnel, mpls, and/or
transport port fields even for packets for which it makes no sense.

This has the side effect of generating different megaflow masks for
different packet types, so there will be more than one kind of mask in
the datapath classifier.  This should not make practical difference,
as megaflows should not be disabled when performance is important.

Signed-off-by: Jarno Rajahalme <jrajahalme@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>