util: do not use mount-utils.IsLikelyNotMountPoint anymore

`IsLikelyNotMountPoint()` is an optimized version for `IsMountPoint()`
which can not detect all type of mounts (anymore). The slower
`IsMountPoint()` is more safe to use. This can cause a slight
performance regression in the case there are many mountpoints on the
system, but correctness is more important than speed while mounting.

Fixes: #4633
Signed-off-by: Niels de Vos <ndevos@ibm.com>

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

@@ -0,0 +1,53 @@
+package userns
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"sync"
+)
+
+var inUserNS = sync.OnceValue(runningInUserNS)
+
+// runningInUserNS detects whether we are currently running in a user namespace.
+//
+// This code was migrated from [libcontainer/runc] and based on an implementation
+// from [lcx/incus].
+//
+// [libcontainer/runc]: https://github.com/opencontainers/runc/blob/3778ae603c706494fd1e2c2faf83b406e38d687d/libcontainer/userns/userns_linux.go#L12-L49
+// [lcx/incus]: https://github.com/lxc/incus/blob/e45085dd42f826b3c8c3228e9733c0b6f998eafe/shared/util.go#L678-L700
+func runningInUserNS() bool {
+	file, err := os.Open("/proc/self/uid_map")
+	if err != nil {
+		// This kernel-provided file only exists if user namespaces are supported.
+		return false
+	}
+	defer file.Close()
+
+	buf := bufio.NewReader(file)
+	l, _, err := buf.ReadLine()
+	if err != nil {
+		return false
+	}
+
+	return uidMapInUserNS(string(l))
+}
+
+func uidMapInUserNS(uidMap string) bool {
+	if uidMap == "" {
+		// File exist but empty (the initial state when userns is created,
+		// see user_namespaces(7)).
+		return true
+	}
+
+	var a, b, c int64
+	if _, err := fmt.Sscanf(uidMap, "%d %d %d", &a, &b, &c); err != nil {
+		// Assume we are in a regular, non user namespace.
+		return false
+	}
+
+	// As per user_namespaces(7), /proc/self/uid_map of
+	// the initial user namespace shows 0 0 4294967295.
+	initNS := a == 0 && b == 0 && c == 4294967295
+	return !initNS
+}