x86/nmi: Upgrade NMI backtrace stall checks & messages

The commit to improve NMI stall debuggability:

  344da544

 ("x86/nmi: Print reasons why backtrace NMIs are ignored")

... has shown value, but widespread use has also identified a few
opportunities for improvement.

The systems have (as usual) shown far more creativity than that commit's
author, demonstrating yet again that failing CPUs can do whatever they want.

In addition, the current message format is less friendly than one might
like to those attempting to use these messages to identify failing CPUs.

Therefore, separately flag CPUs that, during the full time that the
stack-backtrace request was waiting, were always in an NMI handler,
were never in an NMI handler, or exited one NMI handler.

Also, split the message identifying the CPU and the time since that CPU's
last NMI-related activity so that a single line identifies the CPU without
any other variable information, greatly reducing the processing overhead
required to identify repeat-offender CPUs.

Co-developed-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/ab4d70c8-c874-42dc-b206-643018922393@paulmck-laptop