openvswitch: Honor dp_ifindex, when specified, for vport lookup by name.

When OVS_VPORT_ATTR_NAME is specified and dp_ifindex is nonzero, the
logical behavior would be for the vport name lookup scope to be limited
to the specified datapath, but in fact the dp_ifindex value was ignored.
This commit causes the search scope to be honored.

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