rx: wake up send after 'twind' has been updated

rx: wake up send after 'twind' has been updated

Beginning in AFS 3.4 and 3.5 the ack trailer includes the size of the
peer's receive window.  This value is used to update the sender's
transmit window (twind).  When the twind is increased the application
thread is signaled to indicate that more packets can be sent.

This change wakes the application thread after twind is updated by
the peer's receive window instead of beforehand.  Failure to do so
can result in 100ms transmit delays when the receive window transitions
from closed to open.

Reviewed-on: https://gerrit.openafs.org/12625
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit aaa47dc1077f0dd5b0040006c831f64cc8a303b5)

Change-Id: Icfbe10f93a34adfb14f5c34198f78b67aa043c53
Reviewed-on: https://gerrit.openafs.org/12627
Tested-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>