added the exclusion between ADD_PARTITION/DELETE_PARTITION/open() (BLKPG
ioctls didn't grab ->bd_sem when they should have).

added bdev->bd_part; it is set at open() to point to the hd_struct of
partition in question, reset on final close.

blk_partition_remap() uses ->bd_part instead of the current mess

  ->bd_offset is gone, we use ->bd_part->start_sect instead

added missing ->release() to hd_struct kobject, moved kfree() into it

  ->bd_part cotributes to refcount of hd_struct - we bump it when

  ->bd_part is set and drop when it's reset.