archive/tar: optimize formatPAXRecord() call

By replacing fmt.Sprintf with a simple string concat, we see
pretty good improvements across the board on time and memory.

name             old time/op    new time/op    delta
FormatPAXRecord     683ns ± 2%     210ns ± 5%  -69.22%  (p=0.000 n=10+10)

name             old alloc/op   new alloc/op   delta
FormatPAXRecord      112B ± 0%       32B ± 0%  -71.43%  (p=0.000 n=10+10)

name             old allocs/op  new allocs/op  delta
FormatPAXRecord      8.00 ± 0%      2.00 ± 0%  -75.00%  (p=0.000 n=10+10)

Ran with - -cpu=1 -count=10 on an AMD64 i5-5200U CPU @ 2.20GHz

Using the following benchmark:
func BenchmarkFormatPAXRecord(b *testing.B) {
  for n := 0; n < b.N; n++ {
    formatPAXRecord("foo", "bar")
  }
}

Change-Id: I828ddbafad2e5d937f0cf5f777b512638344acfc
Reviewed-on: https://go-review.googlesource.com/55210Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/archive/tar/strconv.go

@@ -6,7 +6,6 @@ package tar
 
 import (
 	"bytes"
-	"fmt"
 	"strconv"
 	"strings"
 	"time"
@@ -266,12 +265,12 @@ func formatPAXRecord(k, v string) (string, error) {
 	const padding = 3 // Extra padding for ' ', '=', and '\n'
 	size := len(k) + len(v) + padding
 	size += len(strconv.Itoa(size))
-	record := fmt.Sprintf("%d %s=%s\n", size, k, v)
+	record := strconv.Itoa(size) + " " + k + "=" + v + "\n"
 
 	// Final adjustment if adding size field increased the record size.
 	if len(record) != size {
 		size = len(record)
-		record = fmt.Sprintf("%d %s=%s\n", size, k, v)
+		record = strconv.Itoa(size) + " " + k + "=" + v + "\n"
 	}
 	return record, nil
 }