rcu: Add force_quiescent_state() testing to rcutorture

Add force_quiescent_state() testing to rcutorture, with a
separate thread that repeatedly invokes force_quiescent_state()
in bursts. This can greatly increase the probability of
encountering certain types of race conditions.

Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <1262646551116-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>