rcu: make CPU stall warning timeout configurable

Also set the default to 60 seconds, up from the previous hard-coded timeout
of 10 seconds.  This allows people who care to set short timeouts, while
avoiding people with unusual configurations (make randconfig!!!) from being
bothered with spurious CPU stall warnings.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>