With ListBuckets() access-list only buckets the user has access (#8037)

This is a behavior change from AWS S3, but it is done with
better judgment on our end to allow the listing of buckets only
which user has access to.

The advantage is this declutters the UI for users and only
lists bucket which they have access to.

Precursor for this feature to be applicable is a policy
must have the following actions

```
s3:ListAllMyBuckets
```
and
```
s3:ListBucket
```

enabled in the policy.

cmd/bucket-handlers.go

@@ -38,6 +38,7 @@ import (
 	"github.com/minio/minio/pkg/event"
 	"github.com/minio/minio/pkg/handlers"
 	"github.com/minio/minio/pkg/hash"
+	iampolicy "github.com/minio/minio/pkg/iam/policy"
 	"github.com/minio/minio/pkg/policy"
 	"github.com/minio/minio/pkg/sync/errgroup"
 )
@@ -202,7 +203,8 @@ func (api objectAPIHandlers) ListBucketsHandler(w http.ResponseWriter, r *http.R
 
 	listBuckets := objectAPI.ListBuckets
 
-	if s3Error := checkRequestAuthType(ctx, r, policy.ListAllMyBucketsAction, "", ""); s3Error != ErrNone {
+	accessKey, owner, s3Error := checkRequestAuthTypeToAccessKey(ctx, r, policy.ListAllMyBucketsAction, "", "")
+	if s3Error != ErrNone {
 		writeErrorResponse(ctx, w, errorCodes.ToAPIErr(s3Error), r.URL, guessIsBrowserReq(r))
 		return
 	}
@@ -236,8 +238,28 @@ func (api objectAPIHandlers) ListBucketsHandler(w http.ResponseWriter, r *http.R
 		}
 	}
 
+	// Set prefix value for "s3:prefix" policy conditionals.
+	r.Header.Set("prefix", "")
+
+	// Set delimiter value for "s3:delimiter" policy conditionals.
+	r.Header.Set("delimiter", SlashSeparator)
+
+	var newBucketsInfo []BucketInfo
+	for _, bucketInfo := range bucketsInfo {
+		if globalIAMSys.IsAllowed(iampolicy.Args{
+			AccountName:     accessKey,
+			Action:          iampolicy.ListBucketAction,
+			BucketName:      bucketInfo.Name,
+			ConditionValues: getConditionValues(r, "", accessKey),
+			IsOwner:         owner,
+			ObjectName:      "",
+		}) {
+			newBucketsInfo = append(newBucketsInfo, bucketInfo)
+		}
+	}
+
 	// Generate response.
-	response := generateListBucketsResponse(bucketsInfo)
+	response := generateListBucketsResponse(newBucketsInfo)
 	encodedSuccessResponse := encodeResponse(response)
 
 	// Write response.