datapath: Change dp_idx to dp_ifindex, the ifindex of the local port.

datapath: Change dp_idx to dp_ifindex, the ifindex of the local port.

I can't see any real value in maintaining a dp_idx separate from the
ifindex of the local port.  With the current implementation it also
artificially limits the number of datapaths.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>