fat: fix listing the root directory

fat: fix listing the root directory

ls / for a FAT partition leads to

   error: invalid modification timestamp for /.

Not all entries of the directory are displayed.

Linux never updates the modification timestamp of the /. directory entry.
The FAT specification allows the access and creation date fields to be
zero.

We should follow Linux and render initial FAT timestamps as start of
the epoch.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Origin: https://lists.gnu.org/archive/html/grub-devel/2022-01/msg00116.html

Gbp-Pq: Name fat-fix-listing-the-root-directory.patch