rcu: Check for jiffies going backwards

A report of a 12-jiffy normal RCU CPU stall warning raises interesting
questions about the nature of time on the offending system.  This commit
instruments rcu_sched_clock_irq(), which is RCU's hook into the
scheduling-clock interrupt, checking for the jiffies counter going
backwards.

Reported-by: Saravanan D <sarvanand@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>