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>

mm/page_alloc.c

@@ -380,20 +380,6 @@ void prep_compound_page(struct page *page, unsigned long order)
 	}
 }
 
-static inline void prep_zero_page(struct page *page, unsigned int order,
-							gfp_t gfp_flags)
-{
-	int i;
-
-	/*
-	 * clear_highpage() will use KM_USER0, so it's a bug to use __GFP_ZERO
-	 * and __GFP_HIGHMEM from hard or soft interrupt context.
-	 */
-	VM_BUG_ON((gfp_flags & __GFP_HIGHMEM) && in_interrupt());
-	for (i = 0; i < (1 << order); i++)
-		clear_highpage(page + i);
-}
-
 #ifdef CONFIG_DEBUG_PAGEALLOC
 unsigned int _debug_guardpage_minorder;
 bool _debug_pagealloc_enabled __read_mostly;
@@ -975,7 +961,8 @@ static int prep_new_page(struct page *page, unsigned int order, gfp_t gfp_flags,
 	kasan_alloc_pages(page, order);
 
 	if (gfp_flags & __GFP_ZERO)
-		prep_zero_page(page, order, gfp_flags);
+		for (i = 0; i < (1 << order); i++)
+			clear_highpage(page + i);
 
 	if (order && (gfp_flags & __GFP_COMP))
 		prep_compound_page(page, order);