dpif-netdev: Avoid copying netdev_flow_key in emc_processing().

dpif-netdev: Avoid copying netdev_flow_key in emc_processing().

Before this commit, emc_processing() copied a netdev_flow_key if there was
no exact-match cache (EMC) hit.  This commit eliminates the copy by
constructing the netdev_flow_key in the place it would be copied.

Found by inspection.

Shahbaz (CCed) reports that this reduces the cost of an EMC miss by 72
cycles in his test case in which the EMC is disabled.  Presumably this
is similarly valuable in cases where the EMC merely has few hits.

For the original version of this patch, which was against a slightly
earlier version of OVS, Daniele reported that:

    - With EMC disabled, this increases throughput from 4.8 Mpps to 5.4
      Mpps.

    - With EMC enabled, this decreases throughput from 12.4 to 12.0 Mpps.

CC: Muhammad Shahbaz <mshahbaz@cs.princeton.edu>
Signed-off-by: Ben Pfaff <blp@ovn.org>
Acked-by: Andy Zhou <azhou@ovn.org>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>