mm/page_idle: constify 'struct bin_attribute'

The sysfs core now allows instances of 'struct bin_attribute' to be moved
into read-only memory.  Make use of that to protect them against
accidental or malicious modifications.

Link: https://lkml.kernel.org/r/20241216-sysfs-const-bin_attr-page_idle-v1-1-cc01ecc55196@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>