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>

net/rxrpc/conn_client.c

@@ -881,7 +881,7 @@ void rxrpc_disconnect_client_call(struct rxrpc_call *call)
 			conn->cache_state = RXRPC_CONN_CLIENT_ACTIVE;
 			rxrpc_activate_channels_locked(conn);
 		}
-		/* fall through */
+		fallthrough;
 	case RXRPC_CONN_CLIENT_ACTIVE:
 		if (list_empty(&conn->waiting_calls)) {
 			rxrpc_deactivate_one_channel(conn, channel);