RDMA/rtrs-clt: Follow "one entry one value" rule for IO migration stats

This commit divides the sysfs entry cpu_migration into 2 different entries
One for "from cpus" and the other for "to cpus".

Link: https://lore.kernel.org/r/20210922125333.351454-8-haris.iqbal@ionos.com


Signed-off-by: Md Haris Iqbal <haris.iqbal@ionos.com>
Reviewed-by: Gioh Kim <gi-oh.kim@ionos.com>
Reviewed-by: Aleksei Marov <aleksei.marov@ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>