selftests: net: fix cleanup_v6() for arp_ndisc_evict_nocarrier

The cleanup_v6() will cause the arp_ndisc_evict_nocarrier script exit
with 255 (No such file or directory), even the tests are good:

 # selftests: net: arp_ndisc_evict_nocarrier.sh
 # run arp_evict_nocarrier=1 test
 # RTNETLINK answers: File exists
 # ok
 # run arp_evict_nocarrier=0 test
 # RTNETLINK answers: File exists
 # ok
 # run all.arp_evict_nocarrier=0 test
 # RTNETLINK answers: File exists
 # ok
 # run ndisc_evict_nocarrier=1 test
 # ok
 # run ndisc_evict_nocarrier=0 test
 # ok
 # run all.ndisc_evict_nocarrier=0 test
 # ok
 not ok 1 selftests: net: arp_ndisc_evict_nocarrier.sh # exit=255

This is because it's trying to modify the parameter for ipv4 instead.

Also, tests for ipv6 (run_ndisc_evict_nocarrier_enabled() and
run_ndisc_evict_nocarrier_disabled() are working on veth1, reflect
this fact in cleanup_v6().

Fixes: f86ca07e ("selftests: net: add arp_ndisc_evict_nocarrier")
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>