os/LevelDBStore: handle deletion race when checking store size

This fixes the fix in 64774e5792f136df2bc78db686440fc2f3a7643f which mixed
up the return value and errno.

Fixes: #6550
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Eduardo Luis <joao.luis@inktank.com>
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>