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>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -718,7 +718,6 @@ static void igb_cache_ring_register(struct igb_adapter *adapter)
 	case e1000_i354:
 	case e1000_i210:
 	case e1000_i211:
-		fallthrough;
 	default:
 		for (; i < adapter->num_rx_queues; i++)
 			adapter->rx_ring[i]->reg_idx = rbase_offset + i;