CVE-2026-53403: fbdev: Fix fb_new_modelist to prevent null-ptr-deref in fb_videomode_to_var

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

fbdev: Fix fbnewmodelist to prevent null-ptr-deref in fbvideomodetovar

info->var, a framebuffer's current mode, is expected to have a matching entry in info->modelist. vartodisplay() relies on this and treats a failed fbmatchmode() as "This should not happen". fbsetvar() keeps it true by adding the mode to the list on every change, and doregisterframebuffer() does the same at registration.

storemodes() replaces the modelist from userspace. fbnewmodelist() validates the new modes but does not check that info->var still has a match. It relies on fbconnewmodelist() to re-point consoles, but that only handles consoles mapped to the framebuffer. With fbcon unbound there are none, so info->var is left describing a mode that is no longer in the list.

A later console takeover runs vartodisplay(), where fbmatchmode() returns NULL and leaves fbdisplay[i].mode NULL. fbconswitch() passes it to displaytovar(), and fbvideomodetovar() dereferences the NULL mode.

Keep the current mode in the list in fbnewmodelist(), the same way fbsetvar() does.