init: Migrate to Go Modules

Moves vendoring over to Go modules.
Fixes issues found by Go Vet in Go 1.16

Signed-off-by: Dave Tucker <dave@dtucker.co.uk>

pkg/init/vendor/github.com/vishvananda/netlink/fou_unspecified.go

@@ -0,0 +1,15 @@
+// +build !linux
+
+package netlink
+
+func FouAdd(f Fou) error {
+	return ErrNotImplemented
+}
+
+func FouDel(f Fou) error {
+	return ErrNotImplemented
+}
+
+func FouList(fam int) ([]Fou, error) {
+	return nil, ErrNotImplemented
+}