rx: Don't call gettimeofday for every packet ack

rx: Don't call gettimeofday for every packet ack

Every time we receive an ACK packet, we call gettimeofday() for
every entry in the transmit queue that's permanently ack'd by that
packet. Instead, just make a note of the time when we start
processing the packet queue, and use it for every packet in the
queue.

This shaves around 5% off rxperf's runtime with a window size of 128.

Reviewed-on: http://gerrit.openafs.org/2956
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit e16d7b7d6a6980fbda3186a578fb7e26fc334194)
Change-Id: Ib77ea09f2b2334ccae269a5e24519cd8b0f0875a
Reviewed-on: http://gerrit.openafs.org/3059
Tested-by: Derrick Brashear <shadow@dementia.org>