ofproto-dpif-xlate: More robust wildcarding for select group.

The flow key should be the same regardless of whether a live bucket is
found or not, as it would be confusing that the flow key would be
different (different mask bits) after the last group bucket goes dead.

In general, the megaflow algorithm expects the mask bits be set as
soon as we read the header bits, regardless of what happens
afterwards.

Also, use flow_mask_hash_fields() instead of individually setting mask
fields. This immediately brings in IPv6 support, and helps keeping
masks in sync with potential algorithm changes to flow hashing
functions.

Found by inspection.

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