dpif-netdev: Delay packets' metadata initialization.

dpif-netdev: Delay packets' metadata initialization.

When a group of packets arrives from a port, we loop through them to
initialize metadata and then we loop through them again to extract the
flow and perform the exact match classification.

This commit combines the two loops into one, and initializes packet->md
in emc_processing() to improve performance.

Since emc_processing() might also be called after recirculation (in
which case the metadata is already valid), an extra parameter is added
to support both cases.

This commits also implements simple prefetching of packet metadata,
to further improve performance.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Andy Zhou <azhou@ovn.org>
Acked-by: Chandran, Sugesh <sugesh.chandran@intel.com>