[#227] netmap: Pre-calculate resulting slice capacity in `flattenNodes`

```
name           old time/op  new time/op  delta
ManySelects-12  17.5µs ±27%   15.5µs ±24%  -11.57%  (p=0.024 n=20+20)

name           old alloc/op  new alloc/op  delta
ManySelects-12  9.03kB ± 0%   8.65kB ± 0%  -4.25%  (p=0.000 n=20+20)

name           old allocs/op  new allocs/op  delta
ManySelects-12  86.0 ± 0%     82.0 ± 0%     -4.65%  (p=0.000 n=20+20)
```

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

netmap/netmap.go

@@ -21,7 +21,14 @@ func NewNetmap(nodes Nodes) (*Netmap, error) {
 }
 
 func flattenNodes(ns []Nodes) Nodes {
-	result := make(Nodes, 0, len(ns))
+	var sz, i int
+
+	for i = range ns {
+		sz += len(ns[i])
+	}
+
+	result := make(Nodes, 0, sz)
+
 	for i := range ns {
 		result = append(result, ns[i]...)
 	}