fs/ntfs3: Fix attr_punch_hole() null pointer derenference

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: NAlon Zahavi <zahavi.alon@gmail.com>
Co-developed-by: NTal Lossos <tallossos@gmail.com>
Signed-off-by: NTal Lossos <tallossos@gmail.com>
Signed-off-by: NKonstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/attrib.c

@@ -2038,7 +2038,7 @@ int attr_punch_hole(struct ntfs_inode *ni, u64 vbo, u64 bytes, u32 *frame_size)
 		return -ENOENT;
 
 	if (!attr_b->non_res) {
-		u32 data_size = le32_to_cpu(attr->res.data_size);
+		u32 data_size = le32_to_cpu(attr_b->res.data_size);
 		u32 from, to;
 
 		if (vbo > data_size)