Low: fs-lib.sh: Optimize fs-lib.sh based filesystem monitor code

fs-lib.sh's is_alive function attempts to determine if a filesystem
resource is healthy by writing and reading a file from disk. The
method used to pick the file name involves a infinite loop where
we try different file names until we get one that doesn't exist.
We don't need to be doing that... Instead we should create a unique
filename based on the resource's instance name and use that for testing.

Resolves: rhbz#1023099