examples: fix use of ethdev internal device array

examples: fix use of ethdev internal device array

Modify l3fwd and related example apps to use locally defined port_conf
instead of global rte_eth_devices which is not a part of public API.
Apps should not be using internal DPDK data structures directly.

Fixes: 1ef9600b2d20 ("examples/l3fwd: convert to ethdev offloads API")
Fixes: ba8c103d2455 ("examples/l3fwd-acl: convert to new ethdev offloads API")
Fixes: 40df1d7a695d ("examples/l3fwd-power: convert to new ethdev offloads API")
Fixes: 43fc038262b8 ("examples/l3fwd-vf: convert to new ethdev offloads API")
Fixes: 373149c631fe ("examples/performance-thread: convert to new offloads API")
Cc: stable@dpdk.org
Signed-off-by: Marcin Zapolski <marcinx.a.zapolski@intel.com>