ntfs3: Remove an access to page->index

Convert the first page passed to ni_write_frame() to a folio and use
folio_pos() on that instead of open-coding the access to folio->index,
cast & shift.

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