fbdev: clear fb_info->mode before deleting a videomode

In the Linux kernel, the following vulnerability has been resolved: 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.