rcutorture: Occasionally delay readers enough to make RCU force_quiescent_state

rcutorture already delays readers, but never for long enough to
make RCU force a quiescent state.  Add an occasional delay of
50ms.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: akpm@linux-foundation.org
Cc: mathieu.desnoyers@polymtl.ca
Cc: josht@linux.vnet.ibm.com
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
LKML-Reference: <12524504772607-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>