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: b964641e9925 ("selftests: pmtu: Make list_flush_ipv6_exception test more demanding")
Signed-off-by: Thadeu Lima de Souza Cascardo <[e-mail]>
Acked-by: Stefano Brivio <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

tools/testing/selftests/net/pmtu.sh

···
 	done
 	run_cmd ${ns_a} ping -q -M want -i 0.1 -c 2 -s 1800 "${dst2}"
 
-	# Each exception is printed as two lines
-	if [ "$(${ns_a} ip route list cache | wc -l)" -ne 202 ]; then
+	if [ "$(${ns_a} ip -oneline route list cache | wc -l)" -ne 101 ]; then
 		err "  can't list cached exceptions"
 		fail=1
 	fi
···
 		run_cmd ${ns_a} ping -q -M want -i 0.1 -w 1 -s 1800 "${dst_prefix1}${i}"
 	done
 	run_cmd ${ns_a} ping -q -M want -i 0.1 -w 1 -s 1800 "${dst2}"
-	if [ "$(${ns_a} ip -6 route list cache | wc -l)" -ne 101 ]; then
+	if [ "$(${ns_a} ip -oneline -6 route list cache | wc -l)" -ne 101 ]; then
 		err "  can't list cached exceptions"
 		fail=1
 	fi