connector: fix skb double free in cn_rx_skb()

When a skb is delivered to a registered callback, cn_call_callback()
incorrectly returns -ENODEV after freeing the skb, causing cn_rx_skb()
to free the skb a second time.

Reported-by: Eric B Munson <emunson@mgebm.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Tested-by: Eric B Munson <emunson@mgebm.net>
Signed-off-by: David S. Miller <davem@davemloft.net>