mm: clarify why we avoid page_mapcount() for slab pages in dump_page()

Let's add comment on why we skip page_mapcount() for sl[aou]b pages.

Link: http://lkml.kernel.org/r/20160922105532.GB24593@node


Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>