net: mctp i2c: Copy headers if cloned

Use skb_cow_head() prior to modifying the TX SKB. This is necessary
when the SKB has been cloned, to avoid modifying other shared clones.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>
Fixes: f5b8abf ("mctp i2c: MCTP I2C binding driver")
Link: https://patch.msgid.link/20250306-matt-mctp-i2c-cow-v1-1-293827212681@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mctp/mctp-i2c.c

@@ -583,6 +583,7 @@ static int mctp_i2c_header_create(struct sk_buff *skb, struct net_device *dev,
 	struct mctp_i2c_hdr *hdr;
 	struct mctp_hdr *mhdr;
 	u8 lldst, llsrc;
+	int rc;
 
 	if (len > MCTP_I2C_MAXMTU)
 		return -EMSGSIZE;
@@ -593,6 +594,10 @@ static int mctp_i2c_header_create(struct sk_buff *skb, struct net_device *dev,
 	lldst = *((u8 *)daddr);
 	llsrc = *((u8 *)saddr);
 
+	rc = skb_cow_head(skb, sizeof(struct mctp_i2c_hdr));
+	if (rc)
+		return rc;
+
 	skb_push(skb, sizeof(struct mctp_i2c_hdr));
 	skb_reset_mac_header(skb);
 	hdr = (void *)skb_mac_header(skb);