treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <[email protected]>
diff --git a/fs/afs/file.c b/fs/afs/file.c
index 6f6ed16..371d148 100644
--- a/fs/afs/file.c
+++ b/fs/afs/file.c
@@ -311,7 +311,7 @@ int afs_page_filler(void *data, struct page *page)
 	case -ENOBUFS:
 		_debug("cache said ENOBUFS");
 
-		/* fall through */
+		fallthrough;
 	default:
 	go_on:
 		req = kzalloc(struct_size(req, array, 1), GFP_KERNEL);