preload: rmdir "$mountpoint" succeeds on non-empty pool

[Marcin Ślusarz]

$ pf mkdir /tmp/mountpoint/123
$ pf rmdir /tmp/mountpoint
$ ls -ld /tmp/mountpoint
ls: cannot access '/tmp/mountpoint': No such file or directory
$ mkdir /tmp/mountpoint
$ pf ls -l /tmp/mountpoint
total 0
drwxrwxrwx 2 root root 4008 Jun 16 12:32 123

Also rm -r "$mountpoint" removes only the mountpoint directory, but doesn't remove any files.