md: Constify attribute_group structs

The attribute_group structs are never modified, they're only passed to
sysfs_create_group() and sysfs_remove_group(). Make them const to allow
the compiler to put them in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Song Liu <song@kernel.org>
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index f83623a..0ee9aa0 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -6940,7 +6940,7 @@ static struct attribute *raid5_attrs[] =  {
 	&ppl_write_hint.attr,
 	NULL,
 };
-static struct attribute_group raid5_attrs_group = {
+static const struct attribute_group raid5_attrs_group = {
 	.name = NULL,
 	.attrs = raid5_attrs,
 };