mm: fix wrong comments about anon_vma lock

We use rwsem since commit 5a505085f043 ("mm/rmap: Convert the struct
anon_vma::mutex to an rwsem").  And most of comments are converted to
the new rwsem lock; while just 2 more missed from:

	 $ git grep 'anon_vma->mutex'

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h
index bc823c4..deca874 100644
--- a/include/linux/mmu_notifier.h
+++ b/include/linux/mmu_notifier.h
@@ -151,7 +151,7 @@
  * Therefore notifier chains can only be traversed when either
  *
  * 1. mmap_sem is held.
- * 2. One of the reverse map locks is held (i_mmap_mutex or anon_vma->mutex).
+ * 2. One of the reverse map locks is held (i_mmap_mutex or anon_vma->rwsem).
  * 3. No other concurrent thread can access the list (release)
  */
 struct mmu_notifier {