mm: fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a couple of
warnings by explicitly adding a break statement instead of just letting
the code fall through to the next, and by adding a fallthrough
pseudo-keyword in places where the code is intended to fall through.

Link: https://github.com/KSPP/linux/issues/115
Link: https://lkml.kernel.org/r/f5756988b8842a3f10008fbc5b0a654f828920a9.1605896059.git.gustavoars@kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mm_init.c

@@ -173,6 +173,7 @@ static int __meminit mm_compute_batch_notifier(struct notifier_block *self,
 	case MEM_ONLINE:
 	case MEM_OFFLINE:
 		mm_compute_batch(sysctl_overcommit_memory);
+		break;
 	default:
 		break;
 	}

mm/vmscan.c

@@ -1369,6 +1369,7 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 				if (PageDirty(page) || PageWriteback(page))
 					goto keep_locked;
 				mapping = page_mapping(page);
+				fallthrough;
 			case PAGE_CLEAN:
 				; /* try to free the page below */
 			}