libcontainer/userns: migrate to github.com/moby/sys/userns

The userns package was moved to the moby/sys/userns module
at commit 3778ae603c.

This patch deprecates the old location, and adds it as an alias
for the moby/sys/userns package.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

vendor/github.com/moby/sys/userns/userns_linux_fuzzer.go

@@ -0,0 +1,8 @@
+//go:build linux && gofuzz
+
+package userns
+
+func FuzzUIDMap(uidmap []byte) int {
+	_ = uidMapInUserNS(string(uidmap))
+	return 1
+}