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

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 fd1894224407c484 ("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
Reported-by: syzbot <syzbot+5ea725c25d06fb9114c4@syzkaller.appspotmail.com>
Fixes: fd1894224407c484 ("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>