selftests/net: fix grep checking for fib_nexthop_multiprefix

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 <liuhangbin@gmail.com>
Link: https://lore.kernel.org/r/20231213060856.4030084-7-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>