Change ifconfig with ip command, on a system where ifconfig is
not used this script will not work correcly.

Test result with this patchset:

sudo make TARGETS="net" kselftest
....
TAP version 13
1..1
 timeout set to 1500
 selftests: net: route_localnet.sh
 run arp_announce test
 net.ipv4.conf.veth0.route_localnet = 1
 net.ipv4.conf.veth1.route_localnet = 1
 net.ipv4.conf.veth0.arp_announce = 2
 net.ipv4.conf.veth1.arp_announce = 2
 PING 127.25.3.14 (127.25.3.14) from 127.25.3.4 veth0: 56(84)
  bytes of data.
 64 bytes from 127.25.3.14: icmp_seq=1 ttl=64 time=0.038 ms
 64 bytes from 127.25.3.14: icmp_seq=2 ttl=64 time=0.068 ms
 64 bytes from 127.25.3.14: icmp_seq=3 ttl=64 time=0.068 ms
 64 bytes from 127.25.3.14: icmp_seq=4 ttl=64 time=0.068 ms
 64 bytes from 127.25.3.14: icmp_seq=5 ttl=64 time=0.068 ms

 --- 127.25.3.14 ping statistics ---
 5 packets transmitted, 5 received, 0% packet loss, time 4073ms
 rtt min/avg/max/mdev = 0.038/0.062/0.068/0.012 ms
 ok
 run arp_ignore test
 net.ipv4.conf.veth0.route_localnet = 1
 net.ipv4.conf.veth1.route_localnet = 1
 net.ipv4.conf.veth0.arp_ignore = 3
 net.ipv4.conf.veth1.arp_ignore = 3
 PING 127.25.3.14 (127.25.3.14) from 127.25.3.4 veth0: 56(84)
  bytes of data.
 64 bytes from 127.25.3.14: icmp_seq=1 ttl=64 time=0.032 ms
 64 bytes from 127.25.3.14: icmp_seq=2 ttl=64 time=0.065 ms
 64 bytes from 127.25.3.14: icmp_seq=3 ttl=64 time=0.066 ms
 64 bytes from 127.25.3.14: icmp_seq=4 ttl=64 time=0.065 ms
 64 bytes from 127.25.3.14: icmp_seq=5 ttl=64 time=0.065 ms

 --- 127.25.3.14 ping statistics ---
 5 packets transmitted, 5 received, 0% packet loss, time 4092ms
 rtt min/avg/max/mdev = 0.032/0.058/0.066/0.013 ms
 ok
ok 1 selftests: net: route_localnet.sh
...
Signed-off-by: Swarup Laxman Kotiaklapudi <swarupkotikalapudi@gmail.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231023123422.2895-1-swarupkotikalapudi@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>