dpif-netdev: Store miniflow length in exact match cache

dpif-netdev: Store miniflow length in exact match cache

This optimization is done to avoid calling count_1bits(), which, if
the popcnt istruction is not available might is slow. popcnt may not
be available because:

- We are running on old hardware
- (more likely) We're using a generic build (i.e. packaged OVS from a
  distro), not tuned for the specific CPU

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>