ntfs: use sb_bdev_nr_blocks

Use the sb_bdev_nr_blocks helper instead of open coding it and clean up
ntfs_fill_super a bit by moving an assignment a little earlier that has
no negative side effects.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Anton Altaparmakov <anton@tuxera.com>
Link: https://lore.kernel.org/r/20211018101130.1838532-29-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>