fbdev: Prevent possible use-after-free in fb_release()

Most fbdev drivers have issues with the fb_info lifetime, because call to
framebuffer_release() from their driver's .remove callback, rather than
doing from fbops.fb_destroy callback.

Doing that will destroy the fb_info too early, while references to it may
still exist, leading to a use-after-free error.

To prevent this, check the fb_info reference counter when attempting to
kfree the data structure in framebuffer_release(). That will leak it but
at least will prevent the mentioned error.

Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Javier Martinez Canillas <[email protected]>
Reviewed-by: Thomas Zimmermann <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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