net: kcm: fix direct access to bv_len

Minor fix for kcm: code wanting to access the fields inside an skb
frag should use the skb_frag_*() helpers, instead of accessing the
fields directly.
Signed-off-by: Mina Almasry <almasrymina@google.com>
Link: https://lore.kernel.org/r/20240102205959.794513-1-almasrymina@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/kcm/kcmsock.c

@@ -634,7 +634,7 @@ static int kcm_write_msgs(struct kcm_sock *kcm)
 
 		msize = 0;
 		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++)
-			msize += skb_shinfo(skb)->frags[i].bv_len;
+			msize += skb_frag_size(&skb_shinfo(skb)->frags[i]);
 
 		iov_iter_bvec(&msg.msg_iter, ITER_SOURCE,
 			      skb_shinfo(skb)->frags, skb_shinfo(skb)->nr_frags,