rbd: support for single-major device number allocation scheme

This series implements userspace support for krbd's single-major device
number allocation scheme, which is needed to break through the limit of
~230 rbd images mapped at once.

A dependency on libblkid >= 2.17 (part of util-linux) is introduced.

Fixes: http://tracker.ceph.com/issues/5048

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>