fs/squashfs: zero out unused fields in fs_dirent

When reading directories the UEFI sub-system must supply file attributes
and timestamps. These fields will have to be added to struct fs_dirent.
SquashFS should not fill these fields with random data. Ensure that they
are zeroed out.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>