rcu: Avoid double multiply by HZ

The rcu_check_gp_start_stall() function multiplies the return value
from rcu_jiffies_till_stall_check() by HZ, but the units are already
in jiffies.  This commit therefore avoids the need for introduction of
a jiffies-squared unit by removing the extraneous multiplication.

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