bonding: incorrect RX counters for bond interface

bonding: incorrect RX counters for bond interface

show interface does not display the RX counters for the bond
interfaces. It displays rx-no-buf instead.

The problem is VNET_INTERFACE_COUNTER_RX is a combined counter,
not a simple counter. Change the code to use
vlib_increment_combined_counter passing it with n_rx_packets and
n_rx_bytes.

Type: fix
Ticket: VPP-1757

Change-Id: I8121ad7e546447049fa13da62481b6c8f5575bec
Signed-off-by: Steven Luong <sluong@cisco.com>
(cherry picked from commit c94afaade0a8fec724d051a202591f5b37dd0f49)