When a process has its working dir inside the mount,
the only way we notice is that we get EBUSY when trying
to unmount.
We used to lazy-unmount in this case, but this means
pulling the rug from under the process.
For example, bash will start throwing
cd: error retrieving current directory: getcwd: cannot access parent directories: No such file or directory
messages.
Fixes https://github.com/rfjakob/gocryptfs/issues/533
de108d3fc0
