pkg/fileutils: add BenchmarkGetTotalUsedFds

go test -bench ^BenchmarkGetTotalUsedFds$ -run ^$ ./pkg/fileutils/
    goos: linux
    goarch: arm64
    pkg: github.com/docker/docker/pkg/fileutils
    BenchmarkGetTotalUsedFds-5   	  149272	      7896 ns/op	     945 B/op	      20 allocs/op

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

pkg/fileutils/fileutils_test.go

@@ -240,3 +240,10 @@ func TestCreateIfNotExistsFile(t *testing.T) {
 		t.Errorf("Should have been a file, seems it's not")
 	}
 }
+
+func BenchmarkGetTotalUsedFds(b *testing.B) {
+	b.ReportAllocs()
+	for i := 0; i < b.N; i++ {
+		_ = GetTotalUsedFds()
+	}
+}