vsock: fix the error return when an invalid ioctl command is used

Currently when an invalid ioctl command is used the error return
is -EINVAL.  Fix this by returning the correct error -ENOIOCTLCMD.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>