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>