sched/core: Idle_task_exit() shouldn't use switch_mm_irqs_off()

sched/core: Idle_task_exit() shouldn't use switch_mm_irqs_off()

idle_task_exit() can be called with IRQs on x86 on and therefore
should use switch_mm(), not switch_mm_irqs_off().

This doesn't seem to cause any problems right now, but it will
confuse my upcoming TLB flush changes.  Nonetheless, I think it
should be backported because it's trivial.  There won't be any
meaningful performance impact because idle_task_exit() is only
used when offlining a CPU.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Fixes: f98db6013c55 ("sched/core: Add switch_mm_irqs_off() and use it in the scheduler")
Link: http://lkml.kernel.org/r/ca3d1a9fa93a0b49f5a8ff729eda3640fb6abdf9.1497034141.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>