replace strings.Split(N) for strings.Cut() or alternatives

Go 1.18 and up now provides a strings.Cut() which is better suited for
splitting key/value pairs (and similar constructs), and performs better:

```go
func BenchmarkSplit(b *testing.B) {
        b.ReportAllocs()
        data := []string{"12hello=world", "12hello=", "12=hello", "12hello"}
        for i := 0; i < b.N; i++ {
                for _, s := range data {
                        _ = strings.SplitN(s, "=", 2)[0]
                }
        }
}

func BenchmarkCut(b *testing.B) {
        b.ReportAllocs()
        data := []string{"12hello=world", "12hello=", "12=hello", "12hello"}
        for i := 0; i < b.N; i++ {
                for _, s := range data {
                        _, _, _ = strings.Cut(s, "=")
                }
        }
}
```

    BenchmarkSplit
    BenchmarkSplit-10            8244206               128.0 ns/op           128 B/op          4 allocs/op
    BenchmarkCut
    BenchmarkCut-10             54411998                21.80 ns/op            0 B/op          0 allocs/op

While looking at occurrences of `strings.Split()`, I also updated some for alternatives,
or added some constraints; for cases where an specific number of items is expected, I used `strings.SplitN()`
with a suitable limit. This prevents (theoretical) unlimited splits.

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

cmd/containerd-stress/density.go

@@ -206,13 +206,13 @@ func parseStat(data string) (stat Stat, err error) {
 		return stat, fmt.Errorf("invalid stat data: %q", data)
 	}
 
-	parts := strings.SplitN(data[:i], "(", 2)
-	if len(parts) != 2 {
+	val, name, ok := strings.Cut(data[:i], "(")
+	if !ok {
 		return stat, fmt.Errorf("invalid stat data: %q", data)
 	}
 
-	stat.Name = parts[1]
-	_, err = fmt.Sscanf(parts[0], "%d", &stat.PID)
+	stat.Name = name
+	_, err = fmt.Sscanf(val, "%d", &stat.PID)
 	if err != nil {
 		return stat, err
 	}
@@ -220,7 +220,7 @@ func parseStat(data string) (stat Stat, err error) {
 	// parts indexes should be offset by 3 from the field number given
 	// proc(5), because parts is zero-indexed and we've removed fields
 	// one (PID) and two (Name) in the paren-split.
-	parts = strings.Split(data[i+2:], " ")
+	parts := strings.Split(data[i+2:], " ")
 	fmt.Sscanf(parts[22-3], "%d", &stat.StartTime)
 	fmt.Sscanf(parts[4-3], "%d", &stat.PPID)
 	return stat, nil