fs/ntfs3: Fix a couple integer overflows on 32bit systems

On 32bit systems the "off + sizeof(struct NTFS_DE)" addition can
have an integer wrapping issue.  Fix it by using size_add().

Fixes: 82cae269 ("fs/ntfs3: Add initialization of super block")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>