fs: simplify init_page_buffers

No need to convert from bdev to inode and back.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20211018101130.1838532-11-hch@lst.de


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