net/ieee802154: don't warn zero-sized raw_sendmsg()

syzbot is hitting skb_assert_len() warning at __dev_queue_xmit() [1],
for PF_IEEE802154 socket's zero-sized raw_sendmsg() request is hitting
__dev_queue_xmit() with skb->len == 0.

Since PF_IEEE802154 socket's zero-sized raw_sendmsg() request was
able to return 0, don't call __dev_queue_xmit() if packet length is 0.

  ----------
  #include <sys/socket.h>
  #include <netinet/in.h>

  int main(int argc, char *argv[])
  {
    struct sockaddr_in addr = { .sin_family = AF_INET, .sin_addr.s_addr = htonl(INADDR_LOOPBACK) };
    struct iovec iov = { };
    struct msghdr hdr = { .msg_name = &addr, .msg_namelen = sizeof(addr), .msg_iov = &iov, .msg_iovlen = 1 };
    sendmsg(socket(PF_IEEE802154, SOCK_RAW, 0), &hdr, 0);
    return 0;
  }
  ----------

Note that this might be a sign that commit fd18942244 ("bpf: Don't
redirect packets with invalid pkt_len") should be reverted, for
skb->len == 0 was acceptable for at least PF_IEEE802154 socket.

Link: https://syzkaller.appspot.com/bug?extid=5ea725c25d06fb9114c4 [1]
Reported-by: syzbot <syzbot+5ea725c25d06fb9114c4@syzkaller.appspotmail.com>
Fixes: fd18942244 ("bpf: Don't redirect packets with invalid pkt_len")
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Link: https://lore.kernel.org/r/20221005014750.3685555-2-aahringo@redhat.com
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>

net/ieee802154/socket.c

@@ -272,6 +272,10 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
 		err = -EMSGSIZE;
 		goto out_dev;
 	}
+	if (!size) {
+		err = 0;
+		goto out_dev;
+	}
 
 	hlen = LL_RESERVED_SPACE(dev);
 	tlen = dev->needed_tailroom;