Use zero_user_page() instead of open-coding it.

[akpm@linux-foundation.org: kmap-type fixes]
Signed-off-by: NNate Diller <nate.diller@gmail.com>
Acked-by: NAnton Altaparmakov <aia21@cantab.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>