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.go

@@ -144,17 +144,14 @@ func listObjectsCommon(layer ObjectLayer, bucket, prefix, marker, delimiter stri
 	case fsObjects:
 		storage = l.storage
 	}
-
 	// Verify if bucket is valid.
 	if !IsValidBucketName(bucket) {
 		return ListObjectsInfo{}, BucketNameInvalid{Bucket: bucket}
 	}
-
-	// Verify whether the bucket exists.
+	// Verify if bucket exists.
 	if !isBucketExist(storage, bucket) {
 		return ListObjectsInfo{}, BucketNotFound{Bucket: bucket}
 	}
-
 	if !IsValidObjectPrefix(prefix) {
 		return ListObjectsInfo{}, ObjectNameInvalid{Bucket: bucket, Object: prefix}
 	}
@@ -174,6 +171,7 @@ func listObjectsCommon(layer ObjectLayer, bucket, prefix, marker, delimiter stri
 		}
 	}
 
+	// With max keys of zero we have reached eof, return right here.
 	if maxKeys == 0 {
 		return ListObjectsInfo{}, nil
 	}