Fix cv_timedwait timeout

Fix cv_timedwait timeout

Perform the already past expiration time check before updating
cvp->cv_mutex with the provided mutex.  This check only depends
on local state.  Doing it first ensures that cvp->cv_mutex will not
be updated in the timeout case or if it's ever called with an
expire_time <= now.

Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #616