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: 6227efc1a2 ("selftests: rtnetlink.sh: add vxlan and fou test cases")
Signed-off-by: Po-Hsu Lin <po-hsu.lin@canonical.com>
Link: https://lore.kernel.org/r/20201019030928.9859-1-po-hsu.lin@canonical.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/config

@@ -33,3 +33,4 @@ CONFIG_KALLSYMS=y
 CONFIG_TRACEPOINTS=y
 CONFIG_NET_DROP_MONITOR=m
 CONFIG_NETDEVSIM=m
+CONFIG_NET_FOU=m

tools/testing/selftests/net/rtnetlink.sh

@@ -520,6 +520,11 @@ kci_test_encap_fou()
 		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"