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.

Change-Id: I65c705c575c4a470b49390e7efca33c279a3133c
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>