proposed fix for cryfs filesystem issue

[Michael-Gallo]

For issue #1543

Works on my machine for getting rid of the high cpu usage.

I can not seem to find a way to detect a cryfs file system that doesn't rely on syscall

[joelim-work]

I have bad news, it looks like tmpfs filesystems (e.g. /tmp on my machine) also have a block count of 0. Disabling nav.checkDir in this case will mean that it's impossible for lf to update the directory contents from load, period timer, etc., and the only way I can think of is to perform a full reload manually, which is not ideal.

Unless there's some other way to distinguish cryfs mounts, I don't think this kind of patch can be merged, since it will fix this bug but end up causing other issues.