rx: fix mutex leak in error case

rx: fix mutex leak in error case

Reported by Mark Vitale

Reviewed-on: https://gerrit.openafs.org/12771
Reviewed-by: Mark Vitale <mvitale@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
(cherry picked from commit 01bcfd3e14f6ee1faa4b8ce5a7932de37d585fd3)

Change-Id: I4384d6813a5cfb053e6991eb3c157fa59ecfa11b
Reviewed-on: https://gerrit.openafs.org/12777
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>