net: caif: use skb helpers instead of open-coding them

Use existing skb_put_data() and skb_trim() instead of open-coding them,
with the skb_put_data() first so that logically, `skb` still contains the
data to be copied in its data..tail area when skb_put_data() reads it.
This change on its own is a cleanup, and it is also necessary for potential
future integration of skbuffs with things like KASAN.

Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/caif/cfpkt_skbuff.c

@@ -319,16 +319,12 @@ struct cfpkt *cfpkt_append(struct cfpkt *dstpkt,
 		if (tmppkt == NULL)
 			return NULL;
 		tmp = pkt_to_skb(tmppkt);
-		skb_set_tail_pointer(tmp, dstlen);
-		tmp->len = dstlen;
-		memcpy(tmp->data, dst->data, dstlen);
+		skb_put_data(tmp, dst->data, dstlen);
 		cfpkt_destroy(dstpkt);
 		dst = tmp;
 	}
-	memcpy(skb_tail_pointer(dst), add->data, skb_headlen(add));
+	skb_put_data(dst, add->data, skb_headlen(add));
 	cfpkt_destroy(addpkt);
-	dst->tail += addlen;
-	dst->len += addlen;
 	return skb_to_pkt(dst);
 }
 
@@ -359,13 +355,11 @@ struct cfpkt *cfpkt_split(struct cfpkt *pkt, u16 pos)
 	if (skb2 == NULL)
 		return NULL;
 
-	/* Reduce the length of the original packet */
-	skb_set_tail_pointer(skb, pos);
-	skb->len = pos;
+	skb_put_data(skb2, split, len2nd);
+
+	/* Reduce the length of the original packet */
+	skb_trim(skb, pos);
 
-	memcpy(skb2->data, split, len2nd);
-	skb2->tail += len2nd;
-	skb2->len += len2nd;
 	skb2->priority = skb->priority;
 	return skb_to_pkt(skb2);
 }