fs/ntfs3: Redesign legacy ntfs support

1) Make is_legacy_ntfs static inline.
2) Put legacy file_operations under #if IS_ENABLED(CONFIG_NTFS_FS).

Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Christian Brauner <brauner@kernel.org>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>