plugin/cache: remove superfluous allocations in item.toMsg (#7700)

This commit removes superfluous allocations of the Answer, Ns, and Extra
slices when copying a cached a dns.Msg. The allocations are superfluous
because we immediately overwrite the newly copied slices with
filterRRSlice. It also updates filterRRSlice to pre-calculate the size
of the slice being copied into.

Benchmark results:

goos: darwin
goarch: arm64
pkg: github.com/coredns/coredns/plugin/cache
cpu: Apple M4 Pro
                 │ base.10.txt │             new.10.txt             │
                 │   sec/op    │   sec/op     vs base               │
CacheResponse-14   471.1n ± 0%   462.9n ± 2%  -1.74% (p=0.009 n=10)

                 │ base.10.txt │            new.10.txt             │
                 │    B/op     │    B/op     vs base               │
CacheResponse-14    672.0 ± 0%   656.0 ± 0%  -2.38% (p=0.000 n=10)

                 │ base.10.txt │            new.10.txt             │
                 │  allocs/op  │ allocs/op   vs base               │
CacheResponse-14    13.00 ± 0%   12.00 ± 0%  -7.69% (p=0.000 n=10)

Signed-off-by: Charlie Vieth <charlie.vieth@gmail.com>

plugin/cache/cache_test.go

@@ -594,16 +594,25 @@ func BenchmarkCacheResponse(b *testing.B) {
 
 	ctx := context.TODO()
 
+	// Add some answers since these need to be duplicated when
+	// serving a cached response.
+	answer := []dns.RR{
+		test.MX("miek.nl.	3601	IN	MX	1 aspmx.l.google.com."),
+		test.MX("miek.nl.	3601	IN	MX	10 aspmx2.googlemail.com."),
+	}
 	reqs := make([]*dns.Msg, 5)
 	for i, q := range []string{"example1", "example2", "a", "b", "ddd"} {
 		reqs[i] = new(dns.Msg)
 		reqs[i].SetQuestion(q+".example.org.", dns.TypeA)
+		reqs[i].Answer = answer
 	}
+	b.ResetTimer()
 
+	rw := &test.ResponseWriter{}
 	j := 0
 	for b.Loop() {
 		req := reqs[j]
-		c.ServeDNS(ctx, &test.ResponseWriter{}, req)
+		c.ServeDNS(ctx, rw, req)
 		j = (j + 1) % 5
 	}
 }