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/core/skmsg.c

@@ -772,7 +772,6 @@ static void sk_psock_verdict_apply(struct sk_psock *psock,
 		sk_psock_skb_redirect(skb);
 		break;
 	case __SK_DROP:
-		/* fall-through */
 	default:
 out_free:
 		kfree_skb(skb);