phonet: fix building with clang

clang warns about overflowing the data[] member in the struct pnpipehdr:

net/phonet/pep.c:295:8: warning: array index 4 is past the end of the array (which contains 1 element) [-Warray-bounds]
if (hdr->data[4] == PEP_IND_READY)
^ ~
include/net/phonet/pep.h:66:3: note: array 'data' declared here
u8 data[1];

Using a flexible array member at the end of the struct avoids the
warning, but since we cannot have a flexible array member inside
of the union, each index now has to be moved back by one, which
makes it a little uglier.

Signed-off-by: Arnd Bergmann <[e-mail]>
Acked-by: Rémi Denis-Courmont <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/phonet/pep.h

···
 		u8		state_after_reset;	/* reset request */
 		u8		error_code;		/* any response */
 		u8		pep_type;		/* status indication */
-		u8		data[1];
+		u8		data0;			/* anything else */
 	};
+	u8			data[];
 };
-#define other_pep_type		data[1]
+#define other_pep_type		data[0]
 
 static inline struct pnpipehdr *pnp_hdr(struct sk_buff *skb)
 {

net/phonet/pep.c

···
 	ph->utid = 0;
 	ph->message_id = id;
 	ph->pipe_handle = pn->pipe_handle;
-	ph->data[0] = code;
+	ph->error_code = code;
 	return pn_skb_send(sk, skb, NULL);
 }
 
···
 	ph->utid = id; /* whatever */
 	ph->message_id = id;
 	ph->pipe_handle = pn->pipe_handle;
-	ph->data[0] = code;
+	ph->error_code = code;
 	return pn_skb_send(sk, skb, NULL);
 }
 
···
 	struct pnpipehdr *ph;
 	struct sockaddr_pn dst;
 	u8 data[4] = {
-		oph->data[0], /* PEP type */
+		oph->pep_type, /* PEP type */
 		code, /* error code, at an unusual offset */
 		PAD, PAD,
 	};
···
 	ph->utid = oph->utid;
 	ph->message_id = PNS_PEP_CTRL_RESP;
 	ph->pipe_handle = oph->pipe_handle;
-	ph->data[0] = oph->data[1]; /* CTRL id */
+	ph->data0 = oph->data[0]; /* CTRL id */
 
 	pn_skb_get_src_sockaddr(oskb, &dst);
 	return pn_skb_send(sk, skb, &dst);
···
 		return -EINVAL;
 
 	hdr = pnp_hdr(skb);
-	if (hdr->data[0] != PN_PEP_TYPE_COMMON) {
+	if (hdr->pep_type != PN_PEP_TYPE_COMMON) {
 		net_dbg_ratelimited("Phonet unknown PEP type: %u\n",
-				    (unsigned int)hdr->data[0]);
+				    (unsigned int)hdr->pep_type);
 		return -EOPNOTSUPP;
 	}
 
-	switch (hdr->data[1]) {
+	switch (hdr->data[0]) {
 	case PN_PEP_IND_FLOW_CONTROL:
 		switch (pn->tx_fc) {
 		case PN_LEGACY_FLOW_CONTROL:
-			switch (hdr->data[4]) {
+			switch (hdr->data[3]) {
 			case PEP_IND_BUSY:
 				atomic_set(&pn->tx_credits, 0);
 				break;
···
 			}
 			break;
 		case PN_ONE_CREDIT_FLOW_CONTROL:
-			if (hdr->data[4] == PEP_IND_READY)
+			if (hdr->data[3] == PEP_IND_READY)
 				atomic_set(&pn->tx_credits, wake = 1);
 			break;
 		}
···
 	case PN_PEP_IND_ID_MCFC_GRANT_CREDITS:
 		if (pn->tx_fc != PN_MULTI_CREDIT_FLOW_CONTROL)
 			break;
-		atomic_add(wake = hdr->data[4], &pn->tx_credits);
+		atomic_add(wake = hdr->data[3], &pn->tx_credits);
 		break;
 
 	default:
 		net_dbg_ratelimited("Phonet unknown PEP indication: %u\n",
-				    (unsigned int)hdr->data[1]);
+				    (unsigned int)hdr->data[0]);
 		return -EOPNOTSUPP;
 	}
 	if (wake)
···
 {
 	struct pep_sock *pn = pep_sk(sk);
 	struct pnpipehdr *hdr = pnp_hdr(skb);
-	u8 n_sb = hdr->data[0];
+	u8 n_sb = hdr->data0;
 
 	pn->rx_fc = pn->tx_fc = PN_LEGACY_FLOW_CONTROL;
 	__skb_pull(skb, sizeof(*hdr));
···
 		return -ECONNREFUSED;
 
 	/* Parse sub-blocks */
-	n_sb = hdr->data[4];
+	n_sb = hdr->data[3];
 	while (n_sb > 0) {
 		u8 type, buf[6], len = sizeof(buf);
 		const u8 *data = pep_get_sb(skb, &type, &len, buf);
···
 	ph->utid = 0;
 	ph->message_id = PNS_PIPE_REMOVE_REQ;
 	ph->pipe_handle = pn->pipe_handle;
-	ph->data[0] = PAD;
+	ph->data0 = PAD;
 	return pn_skb_send(sk, skb, NULL);
 }
 
···
 	peer_type = hdr->other_pep_type << 8;
 
 	/* Parse sub-blocks (options) */
-	n_sb = hdr->data[4];
+	n_sb = hdr->data[3];
 	while (n_sb > 0) {
 		u8 type, buf[1], len = sizeof(buf);
 		const u8 *data = pep_get_sb(skb, &type, &len, buf);
···
 	ph->utid = 0;
 	if (pn->aligned) {
 		ph->message_id = PNS_PIPE_ALIGNED_DATA;
-		ph->data[0] = 0; /* padding */
+		ph->data0 = 0; /* padding */
 	} else
 		ph->message_id = PNS_PIPE_DATA;
 	ph->pipe_handle = pn->pipe_handle;