datapath: Use DP_MAX_PORTS when no IN_PORT attribute is present.

datapath: Use DP_MAX_PORTS when no IN_PORT attribute is present.

To indicate that a flow is not associated with any particular in port,
userspace may omit the IN_PORT attribute, which the kernel translates
internally to the special value DP_MAX_PORTS. After the megaflows
changes, this was no longer being done, resulting in it using port 0
(the internal port).

This also adopts a wildcarding scheme similar to 802.2 packets where
a mask can be specified for this non-existent key attribute but it
must either be completely wildcarded or completely exact match.

CC: Andy Zhou <azhou@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>