selftests: net: return non-zero for failures reported in arp_ndisc_evict_nocarrier

Return non-zero return value if there is any failure reported in this
script during the test. Otherwise it can only reflect the status of
the last command.

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>