[Bug 2098641] [NEW] FAT filesystem probing fails if a file's timestamp exceeds 2038

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[Impact]
If any file's timestamp under `/boot/efi` exceeds 2038, the FAT filesystem probe will fail, and the system will drop into the GRUB shell.

[Fix]
There is an upstream patch that fixes this issue:
https://git.savannah.gnu.org/cgit/grub.git/commit/grub-core/fs?id=e43f3d93b28cce852c110c7a8e40d8311bcd8bb1

[Test Plan]
1. Prepare an EFI partition in FAT format.
root at jammy:~# lsblk -f
vda                                                                           
├─vda1
│    vfat   FAT32       C34E-C891                                   1G     1% /boot/efi
├─vda2
│    ext4   1.0         0cbb726b-e9ac-47bf-8593-9ec43402076f      1.7G     7% /boot
└─vda3
     LVM2_m LVM2        eHoy3y-L7Fk-J4FQ-Fhlw-XXBR-C3xl-gQDAWo                
  └─ubuntu--vg-ubuntu--lv
     ext4   1.0         78ec8d75-3ce6-4f09-abba-c6278aec36e7     79.7G    13% /

2. Change the timestamp of a directory under /boot/efi to exceed 2038:
touch -d "2038-02-27" /boot/efi/EFI

3. Reboot, and the system will be entered into the grub shell.

[Where problems could occur]
The patch simply bypasses the error logic when the modification time is zero in FAT.
There should be no regression unless the FAT filesystem is unable to handle such scenarios.

** Affects: grub2 (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: grub2-unsigned (Ubuntu)
     Importance: Undecided
         Status: New

** Affects: linux-bluefield (Ubuntu)
     Importance: Undecided
     Assignee: Canonical Kernel Team (canonical-kernel-team)
         Status: New

** Affects: grub2 (Ubuntu Focal)
     Importance: Undecided
         Status: Won't Fix

** Affects: grub2-unsigned (Ubuntu Focal)
     Importance: Undecided
         Status: New

** Affects: linux-bluefield (Ubuntu Focal)
     Importance: Undecided
         Status: New

** Affects: grub2 (Ubuntu Jammy)
     Importance: Undecided
         Status: In Progress

** Affects: grub2-unsigned (Ubuntu Jammy)
     Importance: Undecided
         Status: Fix Released

** Affects: linux-bluefield (Ubuntu Jammy)
     Importance: Undecided
         Status: New


** Tags: kernel-daily-bug patch
-- 
FAT filesystem probing fails if a file's timestamp exceeds 2038
https://bugs.launchpad.net/bugs/2098641
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