fs/ntfs3: Use sb instead of sbi->sb in fill_super

Use sb instead of sbi->sb in fill_super. We have sb so why not use
it. Also makes code more readable.

Signed-off-by: Kari Argillander <kari.argillander@gmail.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>