fat: Hook up sb->s_uuid

Now that we have a standard ioctl for querying the filesystem UUID,
initialize sb->s_uuid so that it works.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Link: https://lore.kernel.org/r/20240207025624.1019754-5-kent.overstreet@linux.dev


Signed-off-by: Christian Brauner <brauner@kernel.org>