page->index already considers node offset. The node_offset calculation
in write_sb_page is useless and confusion.

Cc: Goldwyn Rodrigues <rgoldwyn@suse.com>
Cc: NeilBrown <neilb@suse.com>
Acked-by: NGuoqing Jiang <gqjiang@suse.com>
Signed-off-by: NShaohua Li <shli@fb.com>