rx: Introduce ack_is_valid
Take some of our existing logic for ignoring invalid ACK packets and
split it out into a separate function, ack_is_valid. This just makes
it easier to add more complex logic in here and write longer comments
explaining the decisions.
Note that the bug mentioned regarding the previousPacket field was
introduced in IBM AFS 3.5, and was fixed in OpenAFS in commit
bbf92017
(rx: rxi_ReceiveDataPacket do not set rprev on drop), included in
OpenAFS 1.6.23.
This commit incurs no functional change; it is just code
reorganization.
Change-Id: Idd569c6bc0c475e700935cf86780a04ab24102f4
Reviewed-on: https://gerrit.openafs.org/13874
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>