fbdev: clear fb_info->mode before deleting a videomode

fbdev: clear fb_info->mode before deleting a videomode

fb_set_var() can delete a mode from info->modelist when userspace
passes FB_ACTIVATE_INV_MODE through FBIOPUT_VSCREENINFO. The code
checks that the mode being deleted is not the current info->var and
that fbcon is not using it, but it does not check fb_info->mode.

fb_info->mode may still point into the modelist entry being deleted.
If the entry is freed, later mode sysfs reads through show_mode() can
dereference a stale pointer.

Clear fb_info->mode before calling fb_delete_videomode() when it
matches the mode being removed.

Cc: stable@vger.kernel.org # v7.1+
Signed-off-by: Melbin K Mathew <mlbnkm1@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>