vsock: add support for SIOCOUTQ ioctl

Add support for ioctl(s) in AF_VSOCK.
The only ioctl available is SIOCOUTQ/TIOCOUTQ, which returns the number
of unsent bytes in the socket. This information is transport-specific
and is delegated to them using a callback.

Suggested-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Signed-off-by: Luigi Leonardi <luigi.leonardi@outlook.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>