fat: add KUnit tests for timestamp conversion helpers

fat: add KUnit tests for timestamp conversion helpers

Extend fat_test with coverage for FAT timestamp edge cases that are not
currently exercised.

Add tests for fat_time_unix2fat() clamping at the UTC boundaries and
when time_offset pushes an otherwise valid timestamp outside the FAT
date range. Also cover the NULL time_cs path used by existing callers,
and verify fat_truncate_atime() truncation across timezone offsets.

Link: https://patch.msgid.link/20260315222404.9678-1-aviv.daum@gmail.com
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: avivdaum <aviv.daum@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>