1. Need to replace using block_device bdev in blksnap_volume/device with
   gendisk disk and int partno. This makes code compatible with 4.14 and future kernels
   and has sense since notes to kernel's commit:

   ...
   This way we don't need a block_device structure to submit I/O.  The
   block_device has different life time rules from the gendisk and
   request_queue and is usually only available when the block device node
   is open.  Other callers need to explicitly create one (e.g. the lightnvm
   passthrough code, or the new nvme multipathing code)...

   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit?id=74d46992e0d9dee7f1f376de0d56d31614c8a17a

2. ...

