fbdev: Make support for userspace interfaces configurable

fbdev: Make support for userspace interfaces configurable

Add Kconfig option CONFIG_FB_DEVICE and make the virtual fbdev
device optional. If the new option has not been selected, fbdev
does not create files in devfs, sysfs or procfs.

Most modern Linux systems run a DRM-based graphics stack that uses
the kernel's framebuffer console, but has otherwise deprecated fbdev
support. Yet fbdev userspace interfaces are still present.

The option makes it possible to use the fbdev subsystem as console
implementation without support for userspace. This closes potential
entry points to manipulate kernel or I/O memory via framebuffers. It
also prevents the execution of driver code via ioctl or sysfs, both
of which might allow malicious software to exploit bugs in the fbdev
code.

A small number of fbdev drivers require struct fbinfo.dev to be
initialized, usually for the support of sysfs interface. Make these
drivers depend on FB_DEVICE. They can later be fixed if necessary.

v3:
	* effect -> affect in Kconfig help (Daniel)
v2:
	* set FB_DEVICE default to y (Geert)
	* comment on {get,put}_device() (Sam)
	* Kconfig fixes (Sam)
	* add TODO item about FB_DEVICE dependencies (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230613110953.24176-39-tzimmermann@suse.de