x86/mm/dump_pagetables: Speed up page tables dump for CONFIG_KASAN=y

KASAN fills kernel page tables with repeated values to map several
TBs of the virtual memory to the single kasan_zero_page:
  kasan_zero_p4d ->
    kasan_zero_pud ->
        kasan_zero_pmd->
            kasan_zero_pte->
                kasan_zero_page

Walking the whole KASAN shadow range takes a lot of time, especially
with 5-level page tables. Since we already know that all kasan page tables
eventually point to the kasan_zero_page we could call note_page()
right and avoid walking lower levels of the page tables.
This will not affect the output of the kernel_page_tables file,
but let us avoid spending time in page table walkers:

Before:

  $ time cat /sys/kernel/debug/kernel_page_tables > /dev/null

  real    0m55.855s
  user    0m0.000s
  sys     0m55.840s

After:

  $ time cat /sys/kernel/debug/kernel_page_tables > /dev/null

  real    0m0.054s
  user    0m0.000s
  sys     0m0.054s

Signed-off-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170724152558.24689-1-aryabinin@virtuozzo.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>