block: unexport disk_map_sector_rcu

disk_map_sector_rcu is not used by any modular code.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -372,7 +372,6 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 	}
 	return &disk->part0;
 }
-EXPORT_SYMBOL_GPL(disk_map_sector_rcu);
 
 /*
  * Can be deleted altogether. Later.