md-bitmap: account for mddev->bitmap_info.offset in read_sb_page

Diretly apply mddev->bitmap_info.offset to the sector number to read
instead of doing that in both callers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230615064840.629492-9-hch@lst.de