rocker: fix a neigh entry leak issue

Once we get a neighbour through looking up arp cache or creating a
new one in rocker_port_ipv4_resolve(), the neighbour's refcount is
already taken. But as we don't put the refcount again after it's
used, this makes the neighbour entry leaked.

Suggested-by: Eric Dumazet <edumazet@google.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>