fs/ntfs3: Fix attr_punch_hole() null pointer derenference

commit 6d5c9e79 upstream.

The bug occours due to a misuse of `attr` variable instead of `attr_b`.
`attr` is being initialized as NULL, then being derenfernced
as `attr->res.data_size`.

This bug causes a crash of the ntfs3 driver itself,
If compiled directly to the kernel, it crashes the whole system.

Signed-off-by: Alon Zahavi <zahavi.alon@gmail.com>
Co-developed-by: Tal Lossos <tallossos@gmail.com>
Signed-off-by: Tal Lossos <tallossos@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>