mm/list_lru.c: mark expected switch fall-through

In preparation for enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Link: http://lkml.kernel.org/r/20171020190754.GA24332@embeddedor.com
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/list_lru.c

@@ -221,6 +221,7 @@ restart:
 		switch (ret) {
 		case LRU_REMOVED_RETRY:
 			assert_spin_locked(&nlru->lock);
+			/* fall through */
 		case LRU_REMOVED:
 			isolated++;
 			nlru->nr_items--;