dpif-netdev: Load packet pointer only once in emc_processing()

dpif-netdev: Load packet pointer only once in emc_processing()

For the machines I have access to, Reloading the same pointer from
memory seems to inhibit complier optimization somewhat.

In emc_processing(), using a single packet pointer, instead reloading
it from memory with packets[i], improves performance by 0.3 Mpps (tested
with 10G NIC pushing 64 byte packets, with the base line of 12.2 Mpps).

Besides improving performance, this patch should also improves code
readability.

Signed-off-by: Andy Zhou <azhou@ovn.org>
Acked-by: Daniele Di Proietto <diproiettod@vmware.com>