rdma: Add default counter show support

rdma: Add default counter show support

Show default counter statistics, which are same through the sysfs
interface: /sys/class/infiniband/<dev>/ports/<port>/hw_counters/

Example:
$ rdma stat show link mlx5_2/1
link mlx5_2/1 rx_write_requests 8 rx_read_requests 4 rx_atomic_requests 0
out_of_buffer 0 out_of_sequence 0 duplicate_request 0 rnr_nak_retry_err 0
packet_seq_err 0 implied_nak_seq_err 0 local_ack_timeout_err 0
resp_local_length_error 0 resp_cqe_error 0 req_cqe_error 0
req_remote_invalid_request 0 req_remote_access_errors 0
resp_remote_access_errors 0 resp_cqe_flush_error 0 req_cqe_flush_error 0
rp_cnp_ignored 0 rp_cnp_handled 0 np_ecn_marked_roce_packets 0
np_cnp_sent 0 rx_icrc_encapsulated 0

Signed-off-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>