opr: Assert opr_cv_timedwait return codes

opr: Assert opr_cv_timedwait return codes

Almost all of our opr locking primitives assert that they did not
receive an error from pthreads. opr_cv_timedwait is the exception to
this, since it needs to return two possible codes that aren't
internal failures: 0 and ETIMEDOUT. As a result, most callers assert
that our returned code is one of these two values.

To make opr_cv_timedwait act more like our other locking primitives,
removing the need to add opr_Assert()s everywhere, change
opr_cv_timedwait to check that the returned code is 0 or ETIMEDOUT,
and remove the relevant asserts from its callers.

Change-Id: Ie9a62f2edb23969d66e4ed821af37077bc6400c4
Reviewed-on: https://gerrit.openafs.org/14079
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>