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 <gustavoars@kernel.org>
diff --git a/net/rxrpc/peer_event.c b/net/rxrpc/peer_event.c
index a852f46..be03285 100644
--- a/net/rxrpc/peer_event.c
+++ b/net/rxrpc/peer_event.c
@@ -273,7 +273,7 @@ static void rxrpc_store_error(struct rxrpc_peer *peer,
 	case SO_EE_ORIGIN_ICMP6:
 		if (err == EACCES)
 			err = EHOSTUNREACH;
-		/* Fall through */
+		fallthrough;
 	default:
 		_proto("Rx Received error report { orig=%u }", ee->ee_origin);
 		break;