Fix deadlock from BUG in __schedule_bug()

Change-Id: Ib7f33018c19fe08c251f23408ecacf1e929e8670
sched: Fix __schedule_bug() output when called from an interrupt

If schedule is called from an interrupt handler __schedule_bug()
will call show_regs() with the registers saved during the
interrupt handling done in do_IRQ(). This means we'll see the
registers and the backtrace for the process that was interrupted
and not the full backtrace explaining who called schedule().

This is due to 838225b ("sched: use show_regs() to improve
__schedule_bug() output", 2007-10-24) which improperly assumed
that get_irq_regs() would return the registers for the current
stack because it is being called from within an interrupt
handler. Simply remove the show_reg() code so that we dump a
backtrace for the interrupt handler that called schedule().

[ I ran across this when I was presented with a scheduling while
  atomic log with a stacktrace pointing at spin_unlock_irqrestore().
  It made no sense and I had to guess what interrupt handler could
  be called and poke around for someone calling schedule() in an
  interrupt handler. A simple test of putting an msleep() in
  an interrupt handler works better with this patch because you
  can actually see the msleep() call in the backtrace. ]

Also-reported-by: Chris Metcalf <cmetcalf@tilera.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Cc: Satyam Sharma <satyam@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1332979847-27102-1-git-send-email-sboyd@codeaurora.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>