torture: Move stutter_wait() timeouts to hrtimers

[ Upstream commit 10af4367 ]

In order to gain better race coverage, move the test start/stop
waits in stutter_wait() to torture_hrtimeout_jiffies().

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Stable-dep-of: cca42bd8eb1b ("rcutorture: Fix stuttering races and other issues")
Signed-off-by: Sasha Levin <sashal@kernel.org>