rcu: Improve diagnostics for spurious RCU CPU stall warnings

The current RCU CPU stall warning code will print "Stall ended before
state dump start" any time that the stall-warning code is triggered on
a CPU that has already reported a quiescent state for the current grace
period and if all quiescent states have been reported for the current
grace period.  However, a true stall can result in these symptoms, for
example, by preventing RCU's grace-period kthreads from ever running

This commit therefore checks for this condition, reporting the end of
the stall only if one of the grace-period counters has actually advanced.
Otherwise, it reports the last time that the grace-period kthread made
meaningful progress.  (In normal situations, the grace-period kthread
should make meaningful progress at least every jiffies_till_next_fqs
jiffies.)

Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Miroslav Benes <mbenes@suse.cz>
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Git-commit: 6ccd2ecd422644277b7d8b37222e3af3f43ea9ae
[pdaly@codeaurora.org resolve conflict on gpnum from declining
26cdfedf6a902345f8604ea8e0b7dd2566b37a46]
Change-Id: I03d23a023650cf69986fb6f1032b4122a17daba6
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>