EXP rcu/repro: Make timer kthreads sleep at TASK_UNINTERRUPTIBLE

Using torture_hrtimeout_jiffies() causes the calling task to sleep at
TASK_IDLE, which suppresses hung-task warnings.  This commit therefore
directly invokes schedule_hrtimeout() with the task state instead set
to TASK_UNINTERRUPTIBLE, thus permitting the hung-task warnings to
be emitted.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>