mm/rmap.c: fix outdated comment in page_get_anon_vma()

Replace DESTROY_BY_RCU with SLAB_TYPESAFE_BY_RCU because
SLAB_DESTROY_BY_RCU has been renamed to SLAB_TYPESAFE_BY_RCU by commit
5f0d5a3a ("mm: Rename SLAB_DESTROY_BY_RCU to SLAB_TYPESAFE_BY_RCU")

Link: http://lkml.kernel.org/r/20191017093554.22562-1-miles.chen@mediatek.comSigned-off-by: Miles Chen <miles.chen@mediatek.com>
Cc: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/rmap.c

@@ -477,9 +477,10 @@ void __init anon_vma_init(void)
  * chain and verify that the page in question is indeed mapped in it
  * [ something equivalent to page_mapped_in_vma() ].
  *
- * Since anon_vma's slab is DESTROY_BY_RCU and we know from page_remove_rmap()
- * that the anon_vma pointer from page->mapping is valid if there is a
- * mapcount, we can dereference the anon_vma after observing those.
+ * Since anon_vma's slab is SLAB_TYPESAFE_BY_RCU and we know from
+ * page_remove_rmap() that the anon_vma pointer from page->mapping is valid
+ * if there is a mapcount, we can dereference the anon_vma after observing
+ * those.
  */
 struct anon_vma *page_get_anon_vma(struct page *page)
 {