mm: update comments for struct page.mapping

struct page.mapping can be NULL or points to one object of type
address_space, anon_vma or KSM private structure.

Link: http://lkml.kernel.org/r/1506485067-15954-1-git-send-email-changbin.du@intel.com


Signed-off-by: Changbin Du <changbin.du@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>