rbd: fix feature display for --info

Only include the feature if it is set!

Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
rbd: only set STRIPINGV2 feature when needed

Only set the STRIPINGV2 feature if the striping parameters are non-default.
Specifically, fix the case where the passed-in size and count are == 0.

Fixes: #4710
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #232 from ceph/wip-4710

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>