rx: Fix resend accounting

rx: Fix resend accounting

rxi_Start flagged itself as 'resending' whenever it flushed the
transmit queue due to a resend event. However, it would flush the
entire transmit queue at this point, rather than only transmitting
packets that require a resend. When running with large window sizes
this results an a large number of packets erroneously being marked
as resent.

Instead, let SendXmitList decide whether a packet is being
retransmitted by using the presence of a serial number. This takes
advantage of the fact that a retransmitted packet must be the only
entry in a packet list - we just flag the packet list, instead of
having to maintain counters for each individual packet.

(cherry picked from commit e84193ca2a9cef5a13403d291435eb5ad47f5b41)
Reviewed-on: http://gerrit.openafs.org/3135
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Change-Id: I1c8da9639b9739bf88a925b7af98c25b3eaeda8c
Reviewed-on: http://gerrit.openafs.org/4931
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>