don't require a struct block_device to submit a bio

Hi Jens,

this series removes the need to have a struct block_device ready to
submit a bio.   We basically don't use it anywhere in the block stack
anyway - we always go for the gendisk or request_queue.  The only
exception is partition remapping for which we'll now need an additional
partition index.  This helps with cases where we submit I/O from a
character device (nvme or lightnvm passthrough) or a different block
device (upcoming nvme multipath support).