examples/l3fwd: fix build with exact-match enabled

examples/l3fwd: fix build with exact-match enabled

L3fwd was trying to use an inexistent function "simple_ipv6_fwd_4pkts",
instead it should be "simple_ipv6_fwd_8pkts", and "simple_ipv8_fwd_4pkts"
instead of "simple_ipv4_fwd_8pkts".

clang reports some unused functions, used only for LPM lookup:
examples/l3fwd/main.c:545:1: error: unused function 'send_packetsx4'
examples/l3fwd/main.c:1165:1: error: unused function 'rfc1812_process'

Fixes: 80fcb4d4 ("examples/l3fwd: increase lookup burst size to 8")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Sergio Gonzalez Monroy <sergio.gonzalez.monroy@intel.com>
[Thomas: more #if to fix clang warnings]
Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>