posix: ReadAll should handle the case when parent is not a dir. (#2163)

It can happen so that a read request can come for a file which
already has a parent i.e a file.

This fix handles this scenario - fixes #2047

xl-v1-metadata.go

@@ -203,6 +203,7 @@ var objMetadataOpIgnoredErrs = []error{
 	errDiskAccessDenied,
 	errFaultyDisk,
 	errVolumeNotFound,
+	errFileAccessDenied,
 }
 
 // readXLMetadata - returns the object metadata `xl.json` content from