archive/tar: preallocate slice from paxHeaders

Preallocate keys slice with the length of paxHeaders map
to prevent slice growth with append operations.

Change-Id: Ic9a927c4eaa775690a4ef912d61dd06f38e11510
Reviewed-on: https://go-review.googlesource.com/23782Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/writer.go

@@ -317,7 +317,7 @@ func (tw *Writer) writePAXHeader(hdr *Header, paxHeaders map[string]string) erro
 	var buf bytes.Buffer
 
 	// Keys are sorted before writing to body to allow deterministic output.
-	var keys []string
+	keys := make([]string, 0, len(paxHeaders))
 	for k := range paxHeaders {
 		keys = append(keys, k)
 	}