ceph_test_rados: fix snap remove vs rollback fix

In commit 55d279b98553ba4542219b126fc7159b20b18b1f we tried to fix a race
between rollback and snap removal, but got the logic wrong: we need to
prevent *snap removal* on in-use snaps, not prevent multiple rollbacks on
the same snap.

Fixes: #6254 (again)
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Samuel Just <sam.just@inktank.com>