rx: Move transmit queue clearing

rx: Move transmit queue clearing

When the client receives a data packet from the server, it means that
the server has completed processing the client's request. This, in turn,
implies that the transmit queue can be cleared. However, we were doing
this with every incoming data packet.

Move the transmit queue clearing to the code which handles the rest of
the data packet, and make the function only run if the transmit queue
is non-empty.

Now that there's no client specific logic in the ReceiveCall section,
clean up this code to reduce duplication.

Change-Id: Ia4f9024720c676cbcc6d8426d4b94a0acded20bc
Reviewed-on: http://gerrit.openafs.org/8301
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>