krbd: support for single-major device number allocation scheme

This series adds support for single-major device number allocation
scheme, which is needed to break through the limit of ~230 rbd images
mapped at once.  The new limit is 4096 and can be relatively easily
upped in the future by giving users control over the number of minor
numbers to reserve for each mapping.

Fixes: http://tracker.ceph.com/issues/5048

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>