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/openvswitch/flow.c

@@ -675,7 +675,7 @@ static int key_extract_l3l4(struct sk_buff *skb, struct sw_flow_key *key)
 			case -EINVAL:
 				memset(&key->ip, 0, sizeof(key->ip));
 				memset(&key->ipv6.addr, 0, sizeof(key->ipv6.addr));
-				/* fall-through */
+				fallthrough;
 			case -EPROTO:
 				skb->transport_header = skb->network_header;
 				error = 0;