selftests: rtnetlink: load fou module for kci_test_encap_fou() test

The kci_test_encap_fou() test from kci_test_encap() in rtnetlink.sh
needs the fou module to work. Otherwise it will fail with:

$ ip netns exec "$testns" ip fou add port 7777 ipproto 47
RTNETLINK answers: No such file or directory
Error talking to the kernel

Add the CONFIG_NET_FOU into the config file as well. Which needs at
least to be set as a loadable module.

Fixes: 6227efc1a20b ("selftests: rtnetlink.sh: add vxlan and fou test cases")
Signed-off-by: Po-Hsu Lin <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/config

···
 CONFIG_TRACEPOINTS=y
 CONFIG_NET_DROP_MONITOR=m
 CONFIG_NETDEVSIM=m
+CONFIG_NET_FOU=m

tools/testing/selftests/net/rtnetlink.sh

···
 		return $ksft_skip
 	fi
 
+	if ! /sbin/modprobe -q -n fou; then
+		echo "SKIP: module fou is not found"
+		return $ksft_skip
+	fi
+	/sbin/modprobe -q fou
 	ip -netns "$testns" fou add port 7777 ipproto 47 2>/dev/null
 	if [ $? -ne 0 ];then
 		echo "FAIL: can't add fou port 7777, skipping test"