vt_ioctl: move io ioctls to a separate function

We create a new vt_io_ioctl here and move there all the IO ioctls.  This
makes vt_ioctl significantly smaller.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200615074910.19267-32-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>