s390/pkey: 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.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Tested-by: Holger Dengler <dengler@linux.ibm.com>
Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Link: https://lore.kernel.org/r/20241211-sysfs-const-bin_attr-s390-v1-5-be01f66bfcf7@weissschuh.net


Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>