getPids - don't recursively traverse every dir

`filepath.Walk` recursively traverses every dir, which is not what is
needed for getPids.
Instead only read the list of dirs in the top level of `/proc`.

```
benchmark              old ns/op     new ns/op     delta
BenchmarkGetPids-4     868684        195522        -77.49%
```

pkg/util/procfs/procfs_linux_test.go

@@ -23,6 +23,7 @@ import (
 	"os"
 	"os/signal"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"syscall"
 	"testing"
@@ -95,3 +96,21 @@ func TestPKill(t *testing.T) {
 		t.Fatalf("timeout waiting for %v", sig)
 	}
 }
+
+func BenchmarkGetPids(b *testing.B) {
+	if runtime.GOOS == "darwin" || runtime.GOOS == "windows" {
+		b.Skipf("not supported on GOOS=%s", runtime.GOOS)
+	}
+
+	re, err := regexp.Compile("(^|/)" + filepath.Base(os.Args[0]) + "$")
+	assert.Empty(b, err)
+
+	for i := 0; i < b.N; i++ {
+		pids := getPids(re)
+
+		b.StopTimer()
+		assert.NotZero(b, pids)
+		assert.Contains(b, pids, os.Getpid())
+		b.StartTimer()
+	}
+}