osd: ignore CRUSH_ITEM_NONE in compat_must_dispatch_immediately

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #2015 from ceph/wip-8610

osd: ignore CRUSH_ITEM_NONE in compat_must_dispatch_immediately

Reviewed-by: Samuel Just <sam.just@inktank.com>