rcu: Use RCU CPU stall timeout for rcu_check_gp_start_stall()

Currently, rcu_check_gp_start_stall() waits for one second after the first
request before complaining that a grace period has not yet started.  This
was desirable while testing the conversion from ->future_gp_needed[] to
->gp_seq_needed, but it is a bit on the hair-trigger side for production
use under heavy load.  This commit therefore makes this wait time be
exactly that of the RCU CPU stall warning, allowing easy adjustment of
both timeouts to suit the distribution or installation at hand.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>