xz: Landlock: Fix error message if input file is a directory.

xz: Landlock: Fix error message if input file is a directory.

If xz is given a directory, it should look like this:

    $ xz /usr/bin
    xz: /usr/bin: Is a directory, skipping

The Landlock rules didn't allow opening directories for reading:

    $ xz /usr/bin
    xz: /usr/bin: Permission denied

The simplest fix was to allow opening directories for reading.
While it's a bit silly to allow it solely for the error message,
it shouldn't make the sandbox significantly weaker.

The single-file use case (like when called from GNU tar) is
still as strict as possible: all Landlock restrictions are
enabled before (de)compression starts.