vhost: fix VHOST_GET_BACKEND_FEATURES ioctl request definition

The _IOC_READ flag fits this ioctl request more because this request
actually only writes to, but doesn't read from userspace.
See NOTEs in include/uapi/asm-generic/ioctl.h for more information.

Fixes: 429711ae ("vhost: switch to use new message format")
Signed-off-by: Gleb Fotengauer-Malinovskiy <glebfm@altlinux.org>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>