Fixes #6823 : Support T and G file system sizes

I'm a tad confused by this logic.  You're saying that all filesystems should have >= 5 gigs of space, unless /var/lib/mongodb exists, and then if that specific directory exists, subtract however much is on the partition that holds /var/lib/mongodb and that should then >= 5 gigs.  

To me it would make more sense and is simpler:

```
    gb_limit = 5 * 1024 * 1024
    mongo_dir = "/var/lib/mongodb"
    total_space = `df -k --total --exclude-type=tmpfs`.split("\n")[-1].split()[3].to_i
    mongo_size = File.directory?(mongo_dir) ? `du -s  /var/lib/mongodb/`.split()[0].to_i : 0
    error_exit(SIZE, 1)  if (total_space + mongo_size) < gb_limit
```

Note i added --exclude-type=tmpfs   as we likely want to ignore tmpfs partitions.  This basically says make sure 5 GB is free excluding the space taken up by /var/lib/mongodb if it exists