VSOCK: Only check error on skb_recv_datagram when skb is NULL

If skb_recv_datagram returns an skb, we should ignore the err
value returned. Otherwise, datagram receives will return EAGAIN
when they have to wait for a datagram.

Acked-by: Adit Ranadive <aditr@vmware.com>
Signed-off-by: Jorgen Hansen <jhansen@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>