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/20210108224650.25872-1-rikard.falkeborn@gmail.com


Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>