ntfs: Replace ERR_PTR(0) with NULL

ntfs: Replace ERR_PTR(0) with NULL

The variable err is confirmed to be 0 and then never reassigned in the
success path. The function then returns with ERR_PTR(err) which just
equals NULL and can be misleading.

Detected by Smatch:
fs/ntfs/namei.c:1091 ntfs_mkdir() warn:
passing zero to 'ERR_PTR'

Signed-off-by: Ethan Tidmore <ethantidmore06@gmail.com>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>