selftests/landlock: Test file truncation support

These tests exercise the following truncation operations:

* truncate() (truncate by path)
* ftruncate() (truncate by file descriptor)
* open with the O_TRUNC flag
* special case: creat(), which is open with O_CREAT|O_WRONLY|O_TRUNC.

in the following scenarios:

* Files with read, write and truncate rights.
* Files with read and truncate rights.
* Files with the truncate right.
* Files without the truncate right.

In particular, the following scenarios are enforced with the test:

* open() with O_TRUNC requires the truncate right, if it truncates a file.
  open() already checks security_path_truncate() in this case,
  and it required no additional check in the Landlock LSM's file_open hook.
* creat() requires the truncate right
  when called with an existing filename.
* creat() does *not* require the truncate right
  when it's creating a new file.
* ftruncate() requires that the file was opened by a thread that had
  the truncate right for the file at the time of open(). (The rights
  are carried along with the opened file.)

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-6-gnoack3000@gmail.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

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