rcu: Make expedited RCU CPU stall warnings detect stall-end races

rcu: Make expedited RCU CPU stall warnings detect stall-end races

If an expedited RCU CPU stall ends just at the stall-warning timeout,
the current code will print an expedited stall-warning message, but one
that doesn't identify any CPUs or tasks causing the stall.  This is most
likely to happen for short-timeout stalls, for example, the 20-millisecond
timeouts that are sometimes used for small embedded devices.  Needless to
say, these semi-empty stall-warning messages can be rather confusing.

One option would be to suppress the stall-warning message entirely in
this case, but the near-miss information can be quite valuable.

Detect this race condition and emits a "INFO: Expedited stall ended
before state dump start" message to clarify matters.

[boqun: Apply feedback from Borislav]

Reported-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>