rx: Limit window size to max acks

rx: Limit window size to max acks

The RX ack packet can only acknowledge 255 packets at once. In the
current implementation, this limits our maximum window size to 255,
as we can't acknowledge any packets we receive outside of that window
size.

Reviewed-on: http://gerrit.openafs.org/2857
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from commit 50920c0384189a6c635ca58c38bf7c942c0446b2)
Change-Id: I523afdacfc2a50239219d6d39cbea58ca22fee2e
Reviewed-on: http://gerrit.openafs.org/2931
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>