parentDirIsObject() to return quickly with inexistant parent (#11204)

Rewrite parentIsObject() function. Currently if a client uploads
a/b/c/d, we always check if c, b, a are actual objects or not.

The new code will check with the reverse order and quickly quit if 
the segment doesn't exist.

So if a, b, c in 'a/b/c' does not exist in the first place, then returns
false quickly.

cmd/erasure-common_test.go

@@ -66,6 +66,10 @@ func TestErasureParentDirIsObject(t *testing.T) {
 			parentIsObject: true,
 			objectName:     objectName,
 		},
+		{
+			parentIsObject: false,
+			objectName:     "new-dir",
+		},
 		{
 			parentIsObject: false,
 			objectName:     "",