9p: Constify static struct v9fs_attr_group

The only usage of v9fs_attr_group is to pass its address to
sysfs_create_group() and sysfs_create_group(), both which takes pointers
to const struct attribute_group. Make it const to allow the compiler to
put it in read-only memory.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Rikard Falkeborn <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>

fs/9p/v9fs.c

@@ -583,7 +583,7 @@ static struct attribute *v9fs_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group v9fs_attr_group = {
+static const struct attribute_group v9fs_attr_group = {
 	.attrs = v9fs_attrs,
 };