os: LevelDBStore: ignore ENOENT files when estimating store size

While iterating over the store files we race against leveldb, which may
be shuffling data around thus removing some files.

By ignoring missing files on stat, we'll get to not account those files
but that's okay -- this is just an estimate.

Fixes: #6178

Signed-off-by: Joao Eduardo Luis <jecluis@gmail.com>