tipc: Use struct_size() helper

Make use of the struct_size() helper instead of an open-coded version
in order to avoid any potential type mistakes.

This code was detected with the help of Coccinelle and, audited and
fixed manually.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tipc/link.c

@@ -1385,12 +1385,12 @@ u16 tipc_get_gap_ack_blks(struct tipc_gap_ack_blks **ga, struct tipc_link *l,
 		p = (struct tipc_gap_ack_blks *)msg_data(hdr);
 		sz = ntohs(p->len);
 		/* Sanity check */
-		if (sz == tipc_gap_ack_blks_sz(p->ugack_cnt + p->bgack_cnt)) {
+		if (sz == struct_size(p, gacks, p->ugack_cnt + p->bgack_cnt)) {
 			/* Good, check if the desired type exists */
 			if ((uc && p->ugack_cnt) || (!uc && p->bgack_cnt))
 				goto ok;
 		/* Backward compatible: peer might not support bc, but uc? */
-		} else if (uc && sz == tipc_gap_ack_blks_sz(p->ugack_cnt)) {
+		} else if (uc && sz == struct_size(p, gacks, p->ugack_cnt)) {
 			if (p->ugack_cnt) {
 				p->bgack_cnt = 0;
 				goto ok;
@@ -1472,7 +1472,7 @@ static u16 tipc_build_gap_ack_blks(struct tipc_link *l, struct tipc_msg *hdr)
 			__tipc_build_gap_ack_blks(ga, l, ga->bgack_cnt) : 0;
 
 	/* Total len */
-	len = tipc_gap_ack_blks_sz(ga->bgack_cnt + ga->ugack_cnt);
+	len = struct_size(ga, gacks, ga->bgack_cnt + ga->ugack_cnt);
 	ga->len = htons(len);
 	return len;
 }
@@ -1521,7 +1521,7 @@ static int tipc_link_advance_transmq(struct tipc_link *l, struct tipc_link *r,
 		gacks = &ga->gacks[ga->bgack_cnt];
 	} else if (ga) {
 		/* Copy the Gap ACKs, bc part, for later renewal if needed */
-		this_ga = kmemdup(ga, tipc_gap_ack_blks_sz(ga->bgack_cnt),
+		this_ga = kmemdup(ga, struct_size(ga, gacks, ga->bgack_cnt),
 				  GFP_ATOMIC);
 		if (likely(this_ga)) {
 			this_ga->start_index = 0;

net/tipc/msg.h

@@ -189,11 +189,9 @@ struct tipc_gap_ack_blks {
 	struct tipc_gap_ack gacks[];
 };
 
-#define tipc_gap_ack_blks_sz(n) (sizeof(struct tipc_gap_ack_blks) + \
-				 sizeof(struct tipc_gap_ack) * (n))
-
 #define MAX_GAP_ACK_BLKS	128
-#define MAX_GAP_ACK_BLKS_SZ	tipc_gap_ack_blks_sz(MAX_GAP_ACK_BLKS)
+#define MAX_GAP_ACK_BLKS_SZ	(sizeof(struct tipc_gap_ack_blks) + \
+				 sizeof(struct tipc_gap_ack) * MAX_GAP_ACK_BLKS)
 
 static inline struct tipc_msg *buf_msg(struct sk_buff *skb)
 {