dpif-netdev: Prevent unsafe access when retrieving meter stats.

dpif-netdev: Prevent unsafe access when retrieving meter stats.

dpif_netdev_meter_get() retrieved a pointer to a meter entry without
holding a lock.  It's possible that another thread could have deleted
that entry between retrieving the pointer and dereferencing the pointer.
This makes the function hold the lock the entire time the meter entry is
needed.

Found by inspection.

Signed-off-by: Justin Pettit <jpettit@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>