rbd: default to single-major device number scheme

It's been 3.5 years, let's turn it on by default.  Support in rbd(8)
utility goes back to pre-firefly, "rbd map" has been loading the module
with single_major=Y ever since.  However, if the module is already
loaded (whether by hand or at boot time), we end up with single_major=N.
Also, some people don't install rbd(8) and use the sysfs interface
directly.

(With single-major=N, a major number is consumed for every mapping,
imposing a limit of ~240 rbd images per host.  single-major=Y allows
mapping thousands of rbd images on a single machine.)

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>