doc/rados/operations: add kernel client procedure to read balancer documentation
As of now, the kernel client does not support `pg-upmap-primary`. I have
added some troubleshooting steps to help users who are unable to
mount images and filesystems with the kernel client while using `pg-upmap-primary`.
Once the feature is supported by the kernel client, users will be able
to perform mounts along with `pg-upmap-primary`.
Fixes: https://tracker.ceph.com/issues/72897 Signed-off-by: Laura Flores <lflores@ibm.com>