landlock: Fix kernel-doc for the nested quiet layer flag

landlock: Fix kernel-doc for the nested quiet layer flag

kernel-doc emits "Excess struct member 'quiet' description in
'landlock_layer'" because "quiet" is a bitfield inside the named nested
struct "flags", but its inline comment used the bare member name
"@quiet:", which kernel-doc attributes to the enclosing landlock_layer.

Use the canonical dotted notation "@flags.quiet:" so kernel-doc resolves
the nested member, and include it in the generated documentation.

Cc: Justin Suess <utilityemal77@gmail.com>
Cc: Tingmao Wang <m@maowtm.org>
Fixes: a260c0055665 ("landlock: Add a place for flags to layer rules")
Link: https://patch.msgid.link/20260703141711.2016964-1-mic@digikod.net
Signed-off-by: Mickaël Salaün <mic@digikod.net>