dpif-netdev: Simple DROP meter implementation.

Meters may be used by any flow, so some kind of locking must be used.
In this version we have an adaptive mutex for each meter, which may
not be optimal for DPDK.  However, this should serve as a basis for
further improvement.

A batch of packets is first tried as a whole, and only if some of the
meter bands are hit, we need to process the packets individually.

Signed-off-by: Jarno Rajahalme <jarno@ovn.org>
Signed-off-by: Andy Zhou <azhou@ovn.org>