rcutorture: Replace schedule_timeout*() 1-jiffy waits with HZ/20

rcutorture: Replace schedule_timeout*() 1-jiffy waits with HZ/20

In the past, spinning on schedule_timeout* with a wait of 1 jiffy has
hung the kernel. See for example d52d3a2bf408 ("torture: Fix hang during
kthread shutdown phase").

This issue recently recurred in torture's stutter code.  The result is
that the function instantly returns and never goes to sleep, preempting
whatever might otherwise make useful forward progress.

To prevent future issues, apply the commit-d52d3a2bf408 fix throughout
rcutorture, moving from a 1-jiffy wait to a 50-millisecond wait.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>