[CONNECTOR]: clean up {,__}cn_rx_skb()

- __cn_rx_skb() does nothing but calls cn_call_callback(), it doesn't
check skb and msg sizes as the comment suggests, but cn_rx_skb() checks
those sizes.

- In cn_rx_skb() Local variable 'len' is not used. 'len' is probably
intended to be passed to skb_pull(), but here skb_pull() is not needed,
instead skb_free() is called.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>