rbd: add support for single-major device number allocation scheme

rbd: add support for single-major device number allocation scheme

With the preparatory commits ("rbd: match against wholedisk device
numbers on unmap" and "rbd: match against both major and minor on unmap
on kernels >= 3.14") in, this amounts to chosing to work with new rbd
bus interfaces (/sys/bus/rbd/{add,remove}_single_major) if they are
available, instead of the old ones (/sys/bus/rbd/{add,remove}).

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>