AFSOP_STOP_RXEVENT sometimes set without wakeup

AFSOP_STOP_RXEVENT sometimes set without wakeup

make sure when AFSOP_STOP_RXEVENT is set, a wakeup is sent on the
variable so waiters realize it's the case

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/162
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Russ Allbery <rra@stanford.edu>
(cherry picked from commit ba27f1b3cb08db66e184682b30d60ff6fc4ce43e)
Change-Id: If2ac96e7c793c8d4c93492625f006abefac634d1
Reviewed-on: http://gerrit.openafs.org/934
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>