rbd: fix the unmap align and granularity size
rbd: fix the unmap align and granularity size

Get the related limitation from the librbd instead.

Signed-off-by: Xiubo Li <d6cd9ec469dfb11098caa077ea0b7b3220fc70dc@redhat.com>

open-iscsi/tcmu-runner,open-iscsi/tcmu-runner