selftests/net: fix grep checking for fib_nexthop_multiprefix

When running fib_nexthop_multiprefix test I saw all IPv6 test failed.
e.g.

]# ./fib_nexthop_multiprefix.sh
TEST: IPv4: host 0 to host 1, mtu 1300 [ OK ]
TEST: IPv6: host 0 to host 1, mtu 1300 [FAIL]

With -v it shows

COMMAND: ip netns exec h0 /usr/sbin/ping6 -s 1350 -c5 -w5 2001:db8:101::1
PING 2001:db8:101::1(2001:db8:101::1) 1350 data bytes
From 2001:db8:100::64 icmp_seq=1 Packet too big: mtu=1300

--- 2001:db8:101::1 ping statistics ---
1 packets transmitted, 0 received, +1 errors, 100% packet loss, time 0ms

Route get
2001:db8:101::1 via 2001:db8:100::64 dev eth0 src 2001:db8:100::1 metric 1024 expires 599sec mtu 1300 pref medium
Searching for:
2001:db8:101::1 from :: via 2001:db8:100::64 dev eth0 src 2001:db8:100::1 .* mtu 1300

The reason is when CONFIG_IPV6_SUBTREES is not enabled, rt6_fill_node() will
not put RTA_SRC info. After fix:

]# ./fib_nexthop_multiprefix.sh
TEST: IPv4: host 0 to host 1, mtu 1300 [ OK ]
TEST: IPv6: host 0 to host 1, mtu 1300 [ OK ]

Fixes: 735ab2f65dce ("selftests: Add test with multiple prefixes using single nexthop")
Signed-off-by: Hangbin Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/fib_nexthop_multiprefix.sh

···
 		echo "Route get"
 		ip -netns h0 -6 ro get ${dst}
 		echo "Searching for:"
-		echo "    ${dst} from :: via ${r1} dev eth0 src ${h0} .* mtu ${mtu}"
+		echo "    ${dst}.* via ${r1} dev eth0 src ${h0} .* mtu ${mtu}"
 		echo
 	fi
 
 	ip -netns h0 -6 ro get ${dst} | \
-	grep -q "${dst} from :: via ${r1} dev eth0 src ${h0} .* mtu ${mtu}"
+	grep -q "${dst}.* via ${r1} dev eth0 src ${h0} .* mtu ${mtu}"
 	rc=$?
 
 	log_test $rc 0 "IPv6: host 0 to host ${i}, mtu ${mtu}"