mm/page_ext.c: fix a comment

I have noticed that the previous macro is #ifndef CONFIG_SPARSEMEM.  I
think the comment of #else should be CONFIG_SPARSEMEM.

Link: https://lkml.kernel.org/r/20211008140312.6492-1-zhangyinan2019@email.szu.edu.cnSigned-off-by: Yinan Zhang <zhangyinan2019@email.szu.edu.cn>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_ext.c

@@ -201,7 +201,7 @@ void __init page_ext_init_flatmem(void)
 	panic("Out of memory");
 }
 
-#else /* CONFIG_FLATMEM */
+#else /* CONFIG_SPARSEMEM */
 
 struct page_ext *lookup_page_ext(const struct page *page)
 {