net/http: initialize Value with File length in cloneMultipartForm

Improve the initialization of the Value map in cloneMultipartForm by
utilizing the length of the File map to optimize memory allocation.

Change-Id: I97ba9e19b2718a75c270e6df21306f4c82656c71
GitHub-Last-Rev: a9683ba
GitHub-Pull-Request: golang#69943
Reviewed-on: https://go-review.googlesource.com/c/go/+/621235
Reviewed-by: Christian Ekrem <christianekrem@gmail.com>
Reviewed-by: Sean Liao <sean@liao.dev>
Reviewed-by: qiu laidongfeng2 <2645477756@qq.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Bypass: Dmitri Shuralyov <dmitshur@golang.org>

Author: 1911860538

src/net/http/clone.go

@@ -68,7 +68,7 @@ func cloneMultipartForm(f *multipart.Form) *multipart.Form {
 		Value: (map[string][]string)(Header(f.Value).Clone()),
 	}
 	if f.File != nil {
-		m := make(map[string][]*multipart.FileHeader)
+		m := make(map[string][]*multipart.FileHeader, len(f.File))
 		for k, vv := range f.File {
 			vv2 := make([]*multipart.FileHeader, len(vv))
 			for i, v := range vv {