XL/fs: Optimize calling isBucketExist() (#1656)

* posix: Avoid using getAllVolumeInfo() in getVolumeDir()

This is necessary compromise to avoid significant slowness this
causes under load. The compromise is also substantial in a way
so that to avoid penalizing common cases v/s special cases.

For buckets with Caps on Unixes, we filter buckets based on the
latest anyways, so this is completely acceptable.

* XL/fs: Change the usage of verification of existance of buckets.

Optimize calling isBucketExists, it is not needed for all call
paths. isBucketExist should be called only for calls which use
temporary volume location for operations, for the rest rely on
the errors returned on their original call path.

Remove usage of filtering as well across all volume names.

object-common-multipart.go

@@ -80,7 +80,6 @@ func newMultipartUploadCommon(storage StorageAPI, bucket string, object string)
 	if !isBucketExist(storage, bucket) {
 		return "", BucketNotFound{Bucket: bucket}
 	}
-
 	// Verify if object name is valid.
 	if !IsValidObjectName(object) {
 		return "", ObjectNameInvalid{Bucket: bucket, Object: object}
@@ -229,7 +228,6 @@ func abortMultipartUploadCommon(storage StorageAPI, bucket, object, uploadID str
 	if !IsValidBucketName(bucket) {
 		return BucketNameInvalid{Bucket: bucket}
 	}
-	// Verify whether the bucket exists.
 	if !isBucketExist(storage, bucket) {
 		return BucketNotFound{Bucket: bucket}
 	}
@@ -371,7 +369,6 @@ func listMultipartUploadsCommon(layer ObjectLayer, bucket, prefix, keyMarker, up
 	case fsObjects:
 		storage = l.storage
 	}
-
 	result := ListMultipartsInfo{}
 	// Verify if bucket is valid.
 	if !IsValidBucketName(bucket) {