virtio/vsock: rest of SOCK_SEQPACKET support

Small updates to make SOCK_SEQPACKET work:
1) Send SHUTDOWN on socket close for SEQPACKET type.
2) Set SEQPACKET packet type during send.
3) Set 'VIRTIO_VSOCK_SEQ_EOR' bit in flags for last
   packet of message.
4) Implement data check function for SEQPACKET.
5) Check for max datagram size.

Signed-off-by: Arseny Krasnov <arseny.krasnov@kaspersky.com>
Signed-off-by: David S. Miller <davem@davemloft.net>