Unistore will dead lock in a tmpfs

[YangKeao]

As the tmpfs will return EINVAL for direct io, so badger memtable flush goroutine will stop to work, then all write on the unistore will be blocked.

The mysql-test join-reorder cannot execute with a tmpfs store on unistore.

I think it's expected to refuse to work on tmpfs, so we could add a check before starting the unistore, and make this issue easier to find.