datapath: Use RCU dereference in vport_get_ifindex().

datapath: Use RCU dereference in vport_get_ifindex().

If we don't have an ifindex for a device (because it is a virtual
port), we fall back to using the ifindex of the local port.
However, we weren't properly dereferencing the vport from the ports
array, so this adds that.  This isn't a real problem though, because
the local port always exists and never changes as long as the
datapath exists.

Found with sparse.

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