zram: fix up permission for the hot_add sysfs file

Commit 75a2d422 ("driver core: class: mark the struct class for
sysfs callbacks as constant") changed the attribute to use
CLASS_ATTR_RO() which changed the permission from 0400 to 0444.  But
this atribute is "special" in that reading it modifies the system state,
so it MUST be set to 0400 so that only root processes can muck around
with it.

Fix this all up, AND document this so that I don't change it again in
3-4 years when I stumble across it and wonder why it's an open-coded
_ATTR() macro.

Reported-by: Denis Efremov <efremov@linux.com>
Fixes: 75a2d422 ("driver core: class: mark the struct class for sysfs callbacks as constant")
Link: https://lore.kernel.org/r/2023041810-angelic-conical-52d8@gregkh


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>