landlock: Minor comments improvements

landlock: Minor comments improvements

This patch contains some small comment changes.  The first three
comments for ruleset.c, I sort of made along the way while working on /
trying to understand Landlock, and the one from ruleset.h was from the
hashtable patch but extracted here.  In fs.c, one comment which I found
would have been helpful to me when reading this.

Signed-off-by: Tingmao Wang <m@maowtm.org>
Link: https://lore.kernel.org/r/20250602134150.67189-1-m@maowtm.org
Link: https://lore.kernel.org/r/20297185fd71ffbb5ce4fec14b38e5444c719c96.1748379182.git.m@maowtm.org
[mic: Squash patches with updated description, cosmetic fixes]
Signed-off-by: Mickaël Salaün <mic@digikod.net>