RDMA/nldev: Allow optional-counter status configuration through RDMA netlink

Provide an option to allow users to enable/disable optional counters
through RDMA netlink. Limiting it to users with ADMIN capability only.

Examples:
1. Enable optional counters cc_rx_ce_pkts and cc_rx_cnp_pkts (and
   disable all others):
$ sudo rdma statistic set link rocep8s0f0/1 optional-counters \
    cc_rx_ce_pkts,cc_rx_cnp_pkts

2. Remove all optional counters:
$ sudo rdma statistic unset link rocep8s0f0/1 optional-counters

Link: https://lore.kernel.org/r/20211008122439.166063-10-markzhang@nvidia.com
Signed-off-by: Aharon Landau <aharonl@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>