samples/landlock: Extend sample tool to support LANDLOCK_ACCESS_FS_TRUNCATE

Update the sandboxer sample to restrict truncate actions. This is
automatically enabled by default if the running kernel supports
LANDLOCK_ACCESS_FS_TRUNCATE, except for the paths listed in the
LL_FS_RW environment variable.

Signed-off-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20221018182216.301684-11-gnoack3000@gmail.com


Signed-off-by: Mickaël Salaün <mic@digikod.net>