VSOCK: Only check error on skb_recv_datagram when skb is NULL

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>
(cherry picked from commit 9c995cc9a206a008699da82f6cd01e9b2615649a)
Signed-off-by: Dan Duval <dan.duval@oracle.com>
(cherry picked from commit 79acddbe36a34e54b558ca7f30938e75e005863b)

Orabug: 23718522

Signed-off-by: Manjunath Govindashetty <manjunath.govindashetty@oracle.com>