net: mctp i3c: 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: c8755b2 ("mctp i3c: MCTP I3C driver")
Link: https://patch.msgid.link/20250306-matt-i3c-cow-head-v1-1-d5e6a5495227@codeconstruct.com.au
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/mctp/mctp-i3c.c

@@ -506,10 +506,15 @@ static int mctp_i3c_header_create(struct sk_buff *skb, struct net_device *dev,
 	   const void *saddr, unsigned int len)
 {
 	struct mctp_i3c_internal_hdr *ihdr;
+	int rc;
 
 	if (!daddr || !saddr)
 		return -EINVAL;
 
+	rc = skb_cow_head(skb, sizeof(struct mctp_i3c_internal_hdr));
+	if (rc)
+		return rc;
+
 	skb_push(skb, sizeof(struct mctp_i3c_internal_hdr));
 	skb_reset_mac_header(skb);
 	ihdr = (void *)skb_mac_header(skb);