RDMA/nldev: Don't expose number of not-visible entries

Netlink dumpit handshake exchanges the index from which kernel should
start to return its value, in current code, this index included
not-visible in this PID items too and indirectly revealed the number of
entries.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>