for partitioned devices we use ->nr_sect to find the size; blk_size[] is
still used for things like floppy.c, etc.; that will go away later.

There was only one place (do_open()) that needed it - the rest uses
->bd_inode->i_size now.  So blkdev_size_in_bytes() is gone - it's
expanded in its only caller.  Same place (do_open()) finds the partition
offset and stores it in new field ->bd_offset.  As the result, call of
get_gendisk() is gone from the IO path - in blk_partition_remap() we
just add ->bd_offset.

Additionally, we take driver probing (get_blkfops()) outside of ->bd_sem
(again, do_open()) - that will allow to kill ad-hackery in check_partitions()
(opening bdev by hand).