rbd: match against both major and minor on unmap on newer kernels

rbd: match against both major and minor on unmap on newer kernels

As described in commit "rbd: match against wholedisk device numbers on
unmap", currently we only match against major numbers.  In preparation
for support for single-major device number allocation scheme, start
matching against minor numbers also, which newer kernels provide in
a /sys/bus/rbd/devices/<id>/minor sysfs attribute.

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