os/LevelDBStore, RocksDBStore: fix race handling for get store size

If we get ENOENT, skip this file, instead of adding in undefined stat
values.

Backport: firefly
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2800 from ceph/wip-enoent-race

os/LevelDBStore, RocksDBStore: fix race handling for get store size

Reviewed-by: Joao Eduardo Luis <joao@redhat.com>