mm: thp: __get_page_tail_foll() can use get_huge_page_tail()

Cleanup. Change __get_page_tail_foll() to use get_huge_page_tail()
to avoid the code duplication.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Dave Jones <davej@redhat.com>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Acked-by: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/internal.h

@@ -47,12 +47,9 @@ static inline void __get_page_tail_foll(struct page *page,
 	 * page_cache_get_speculative()) on tail pages.
 	 */
 	VM_BUG_ON(atomic_read(&page->first_page->_count) <= 0);
-	VM_BUG_ON(atomic_read(&page->_count) != 0);
-	VM_BUG_ON(page_mapcount(page) < 0);
 	if (get_page_head)
 		atomic_inc(&page->first_page->_count);
-	if (compound_tail_refcounted(page->first_page))
-		atomic_inc(&page->_mapcount);
+	get_huge_page_tail(page);
 }
 
 /*