cls_rbd: default initialize snapshot namespace for legacy clients

cls_rbd: default initialize snapshot namespace for legacy clients

Creating a snapshot on >=Kraken OSDs using <=Jewel clients can result
in an improperly initialized snapshot namespace. As a result, attempting
to remove the snapshot using a >=Kraken client will result in an -EINVAL
error.

Fixes: http://tracker.ceph.com/issues/19413
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 03b0b03071f3e04754896664c69f73759ddb907a)

Conflicts:
	src/cls/rbd/cls_rbd.h: trivial resolution