fix: under FanOut API avoid repeated md5sum calculation (#17572)

md5sum calculation has a high CPU overhead, avoid calculating
it repeatedly for similar fanOut calls.

To fix following CPU profiler result
```
(pprof) top10
Showing nodes accounting for 678.68s, 84.67% of 801.54s total
Dropped 1072 nodes (cum <= 4.01s)
Showing top 10 nodes out of 156
      flat  flat%   sum%        cum   cum%
   332.54s 41.49% 41.49%    332.54s 41.49%  runtime/internal/syscall.Syscall6
   228.39s 28.49% 69.98%    228.39s 28.49%  crypto/md5.block
    48.07s  6.00% 75.98%     48.07s  6.00%  runtime.memmove
    28.91s  3.61% 79.59%     28.91s  3.61%  github.com/minio/highwayhash.updateAVX2
     8.25s  1.03% 80.61%      8.25s  1.03%  runtime.futex
     8.25s  1.03% 81.64%     10.81s  1.35%  runtime.step
     6.99s  0.87% 82.52%     22.35s  2.79%  runtime.pcvalue
     6.67s  0.83% 83.35%     38.90s  4.85%  runtime.mallocgc
     5.77s  0.72% 84.07%     32.61s  4.07%  runtime.gentraceback
     4.84s   0.6% 84.67%     10.49s  1.31%  runtime.lock2
```

cmd/bucket-handlers.go

@@ -20,7 +20,9 @@ package cmd
 import (
 	"bytes"
 	"context"
+	"crypto/md5"
 	"encoding/base64"
+	"encoding/hex"
 	"encoding/json"
 	"encoding/xml"
 	"errors"
@@ -1290,15 +1292,20 @@ func (api objectAPIHandlers) PostPolicyBucketHandler(w http.ResponseWriter, r *h
 		buf := bytebufferpool.Get()
 		defer bytebufferpool.Put(buf)
 
+		md5w := md5.New()
+
 		// Maximum allowed fan-out object size.
 		const maxFanOutSize = 16 << 20
 
-		n, err := io.Copy(buf, ioutil.HardLimitReader(pReader, maxFanOutSize))
+		n, err := io.Copy(io.MultiWriter(buf, md5w), ioutil.HardLimitReader(pReader, maxFanOutSize))
 		if err != nil {
 			writeErrorResponse(ctx, w, toAPIError(ctx, err), r.URL)
 			return
 		}
 
+		// Set the correct hex md5sum for the fan-out stream.
+		fanOutOpts.MD5Hex = hex.EncodeToString(md5w.Sum(nil))
+
 		concurrentSize := 100
 		if runtime.GOMAXPROCS(0) < concurrentSize {
 			concurrentSize = runtime.GOMAXPROCS(0)