fat: convert to new timestamp accessors

Convert to using the new inode timestamp accessor functions.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20231004185347.80880-35-jlayton@kernel.org


Tested-by: Klara Modin <klarasmodin@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>