target/iblock: Remove unused iblock_dev members

ibd_depth and ibd_force are used write-only.  Remove them.
ibd_major/minor can be easily retrieved from ibd_bd, so get
rid of them too.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>