Add RDMA nldev netlink interface for dumping MR statistics information.

Output example:

$ ./ibv_rc_pingpong -o -P -s 500000000
  local address:  LID 0x0001, QPN 0x00008a, PSN 0xf81096, GID ::

$ rdma stat show mr
dev mlx5_0 mrn 2 page_faults 122071 page_invalidations 0

Link: https://lore.kernel.org/r/20191016062308.11886-5-leon@kernel.orgSigned-off-by: NErez Alfasi <ereza@mellanox.com>
Signed-off-by: NLeon Romanovsky <leonro@mellanox.com>
Reviewed-by: NJason Gunthorpe <jgg@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>