powerpc/64s: Fix unrecoverable SLB crashes due to preemption check

Hugh reported that his trusty G5 crashed after a few hours under load
with an "Unrecoverable exception 380".

The crash is in interrupt_return() where we check lazy_irq_pending(),
which calls get_paca() and with CONFIG_DEBUG_PREEMPT=y that goes to
check_preemption_disabled() via debug_smp_processor_id().

As Nick explained on the list:

  Problem is MSR[RI] is cleared here, ready to do the last few things
  for interrupt return where we're not allowed to take any other
  interrupts.

  SLB interrupts can happen just about anywhere aside from kernel
  text, global variables, and stack. When that hits, it appears to be
  unrecoverable due to RI=0.

The problematic access is in preempt_count() which is:

	return READ_ONCE(current_thread_info()->preempt_count);

Because of THREAD_INFO_IN_TASK, current_thread_info() just points to
current, so the access is to somewhere in kernel memory, but not on
the stack or in .data, which means it can cause an SLB miss. If we
take an SLB miss with RI=0 it is fatal.

The easiest solution is to add a version of lazy_irq_pending() that
doesn't do the preemption check and call it from the interrupt return
path.

Fixes: 68b34588

 ("powerpc/64/sycall: Implement syscall entry/exit logic in C")
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200502143316.929341-1-mpe@ellerman.id.au