x86/mm/pat: Disable preemption around __flush_tlb_all()

The WARN_ON_ONCE(__read_cr3() != build_cr3()) in switch_mm_irqs_off()
triggers every once in a while during a snapshotted system upgrade.

The warning triggers since commit decab088 ("x86/mm: Remove
preempt_disable/enable() from __native_flush_tlb()"). The callchain is:

  get_page_from_freelist() -> post_alloc_hook() -> __kernel_map_pages()

with CONFIG_DEBUG_PAGEALLOC enabled.

Disable preemption during CR3 reset / __flush_tlb_all() and add a comment
why preemption has to be disabled so it won't be removed accidentaly.

Add another preemptible() check in __flush_tlb_all() to catch callers with
enabled preemption when PGE is enabled, because PGE enabled does not
trigger the warning in __native_flush_tlb(). Suggested by Andy Lutomirski.

Fixes: decab088

 ("x86/mm: Remove preempt_disable/enable() from __native_flush_tlb()")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20181017103432.zgv46nlu3hc7k4rq@linutronix.de