vsock/virtio: set SOCK_DONE on peer shutdown

Set the SOCK_DONE flag to match the TCP_CLOSING state when a peer has
shut down and there is nothing left to read.

This fixes the following bug:
1) Peer sends SHUTDOWN(RDWR).
2) Socket enters TCP_CLOSING but SOCK_DONE is not set.
3) read() returns -ENOTCONN until close() is called, then returns 0.

Signed-off-by: Stephen Barber <smbarber@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>