RDMA/rtrs: Constify static struct attribute_group

The only usage of these is to pass their address to sysfs_create_group()
and sysfs_remove_group(), both which takes const pointers. Make it const
to allow the compiler to put them in read-only memory.

Link: https://lore.kernel.org/r/20200930224004.24279-3-rikard.falkeborn@gmail.com


Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>