mm, hwpoison: fixup "mm: check the return value of lookup_page_ext for all call sites"

Backport of the upstream commit f86e4271978b ("mm: check the return
value of lookup_page_ext for all call sites") is wrong for hwpoison
pages. I have accidentally negated the condition for bailout. This
basically disables hwpoison pages tracking while the code still
might crash on unusual configurations when struct pages do not have
page_ext allocated. The fix is trivial to invert the condition.

Reported-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Michal Hocko

mm/debug-pagealloc.c

@@ -34,7 +34,7 @@ static inline void set_page_poison(struct page *page)
 	struct page_ext *page_ext;
 
 	page_ext = lookup_page_ext(page);
-	if (page_ext)
+	if (!page_ext)
 		return;
 	__set_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
 }
@@ -44,7 +44,7 @@ static inline void clear_page_poison(struct page *page)
 	struct page_ext *page_ext;
 
 	page_ext = lookup_page_ext(page);
-	if (page_ext)
+	if (!page_ext)
 		return;
 	__clear_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
 }
@@ -54,7 +54,7 @@ static inline bool page_poison(struct page *page)
 	struct page_ext *page_ext;
 
 	page_ext = lookup_page_ext(page);
-	if (page_ext)
+	if (!page_ext)
 		return false;
 	return test_bit(PAGE_EXT_DEBUG_POISON, &page_ext->flags);
 }