mm: cleanup the comment for head/tail pages of compound pages in mm/page_alloc.c

Only tail pages point at the head page using their ->first_page fields.
Signed-off-by: NWang Sheng-Hui <shhuiw@gmail.com>
Reviewed-by: NMichal Hocko <mhocko@suse.cz>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

mm/page_alloc.c

@@ -333,8 +333,8 @@ static void bad_page(struct page *page)
  *
  * The remaining PAGE_SIZE pages are called "tail pages".
  *
- * All pages have PG_compound set.  All pages have their ->private pointing at
- * the head page (even the head page has this).
+ * All pages have PG_compound set.  All tail pages have their ->first_page
+ * pointing at the head page.
  *
  * The first tail page's ->lru.next holds the address of the compound page's
  * put_page() function.  Its ->lru.prev holds the order of allocation.