rbd: set the remaining discard properties to enable support
rbd: set the remaining discard properties to enable support

max_discard_sectors must be set for the queue to support discard.
Operations implementing discard for rbd zero data, so report that.

Signed-off-by: Josh Durgin <51652c783f70f5b272ebd02f1fd20d3af7a2449f@inktank.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs