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.

Change-Id: Id129ea93e94612a4b8cce9f8cbddde9c779ff26b
Reviewed-on: https://gerrit.openafs.org/12625
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>