selftests: pmtu: use -oneline for ip route list cache

Some versions of iproute2 will output more than one line per entry, which
will cause the test to fail, like:

TEST: ipv6: list and flush cached exceptions                        [FAIL]
  can't list cached exceptions

That happens, for example, with iproute2 4.15.0. When using the -oneline
option, this will work just fine:

TEST: ipv6: list and flush cached exceptions                        [ OK ]

This also works just fine with a more recent version of iproute2, like
5.4.0.

For some reason, two lines are printed for the IPv4 test no matter what
version of iproute2 is used. Use the same -oneline parameter there instead
of counting the lines twice.

Fixes: b964641e ("selftests: pmtu: Make list_flush_ipv6_exception test more demanding")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Acked-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>