(repost) cleaning up handling of bad IRQs

["Enrico Weigelt, metux IT consult" <info@metux.net>]

Hello friends,

 << reposting, since first queue didn't go through completely, due to mailer problem >>

here's a patch queue for cleaning up the IRQ handling. Inspired by a
discussion we had on a previous patch of mine:

    "arch: fix 'unexpected IRQ trap at vector' warnings"
    https://www.spinics.net/lists/kernel/msg3763137.html

Turned out that the whole message, as it is right now, doesn't make much
sense at at all - not just incorrect wording, but also not quite useful
information. And the whole ack_bad_irq() thing deserves a cleanup anyways.

So, I've had a closer look and came to these conclusions:

1. The warning message doesn't need to be duplicated in the per architecture
   ack_bad_irq() functions. All, but one callers already do their own warning.
   Thus just adding a pr_warn() call there, printing out more useful data
   like the hardware IRQ number, and dropping all warnings from all the
   ack_bad_irq() functions.

2. Many of the ack_bad_irq()'s count up the spurious interrupts - lots of
   duplications over the various archs. Some of them using atomic_t, some
   just plain ints. Consolidating this by introducing a global counter
   with inline'd accessors and doing the upcounting in the (currently 3)
   call sites of ack_bad_irq(). After that, step by step changing all
   archs to use the new counter.

3. For all but one arch (x86), ack_bad_irq() became a no-op.

   On x86, it's just a call to ack_APIC_irq(), in order to prevent lockups
   when IRQs missed to be ack'ed on the APIC. Could we perhaps do this in
   some better place ? In that case, ack_bad_irq() could easily be removed
   entirely.

have fun,

--mtx