rcu: Reduce default RCU CPU stall warning timeout

The RCU CPU stall warning timeout has defaulted to 60 seconds for
some years, with almost no false positives.  This commit therefore
reduces the default to 21 seconds, slightly shorter than the new
soft-lockup timeout.
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

lib/Kconfig.debug

@@ -972,7 +972,7 @@ config RCU_CPU_STALL_TIMEOUT
 	int "RCU CPU stall timeout in seconds"
 	depends on TREE_RCU || TREE_PREEMPT_RCU
 	range 3 300
-	default 60
+	default 21
 	help
 	  If a given RCU grace period extends more than the specified
 	  number of seconds, a CPU stall warning is printed.  If the