oulet/routing: fix RIB lookup benchmark

b.N is not updated with the current loop count. Use a counter for that.
See #351.

```
 08:37 ❱ go test -run='^$' -bench="BenchmarkRIBLookup" akvorado/outlet/routing/provider/bmp
goos: linux
goarch: amd64
pkg: akvorado/outlet/routing/provider/bmp
cpu: AMD Ryzen 5 5600X 6-Core Processor
BenchmarkRIBLookup/1000_routes,_1_peers-12              68083086                16.70 ns/op
BenchmarkRIBLookup/1000_routes,_2_peers-12              70699065                17.29 ns/op
BenchmarkRIBLookup/1000_routes,_5_peers-12              71537038                17.39 ns/op
BenchmarkRIBLookup/10000_routes,_1_peers-12             60138338                18.91 ns/op
BenchmarkRIBLookup/10000_routes,_2_peers-12             60308262                19.59 ns/op
BenchmarkRIBLookup/10000_routes,_5_peers-12             50109068                23.59 ns/op
BenchmarkRIBLookup/100000_routes,_1_peers-12            30180804                39.37 ns/op
BenchmarkRIBLookup/100000_routes,_2_peers-12            26725899                41.08 ns/op
BenchmarkRIBLookup/100000_routes,_5_peers-12            28283074                41.47 ns/op
BenchmarkRIBLookup/1000000_routes,_1_peers-12           15247650                72.55 ns/op
BenchmarkRIBLookup/1000000_routes,_2_peers-12           13945797               101.2 ns/op
BenchmarkRIBLookup/1000000_routes,_5_peers-12            9414166               108.6 ns/op
PASS
ok      akvorado/outlet/routing/provider/bmp    23.461s
```

outlet/routing/provider/bmp/prefixes_test.go

@@ -311,11 +311,13 @@ func BenchmarkRIBLookup(b *testing.B) {
 					}
 				}
 
+				count := 0
 				for b.Loop() {
-					ip4 := randomPrefixes[b.N%len(randomPrefixes)].Prefix.Addr()
+					count++
+					ip4 := randomPrefixes[count%len(randomPrefixes)].Prefix.Addr()
 					_, _ = rib.tree.Lookup(ip4)
 				}
-				b.ReportMetric(float64(b.Elapsed())/float64(b.N), "ns/op")
+				b.ReportMetric(float64(b.Elapsed())/float64(count), "ns/op")
 			})
 		}
 	}