If the backend advertises 'feature-discard', then interrogate
the backend for alignment and granularity. Setup the request
queue with the appropiate values and send the discard operation
as required.
Signed-off-by: NLi Dongyang <lidongyang@novell.com>
[v1: Amended commit description]
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>