SoftIndexFileStore availability check checks wrong file

The SoftIndexFileStore checks the availability of a file. But unfortunately it is a relative path for the configured file https://github.com/infinispan/infinispan/blob/33c0f3961955a473db774199fe30b5b5cef79b09/persistence/soft-index/src/main/java/org/infinispan/persistence/sifs/NonBlockingSoftIndexFileStore.java#L423. It should use the qualified location that other parts use.

This also doesn't take into account that the prior code checked the data location twice