mm/page_alloc.c: cleanup obsolete KM_USER*

It's been five years now that KM_* kmap flags have been removed and that
we can call clear_highpage from any context.  So we remove prep_zero_pages
accordingly.

Signed-off-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>