Make sure to handle FaultyDisks in listing ops (#6204)

Continuing from PR 157ed65c35

Our posix.go implementation did not handle  I/O errors
properly on the disks, this led to situations where
top-level callers such as ListObjects might return early
without even verifying all the available disks.

This commit tries to address this in Kubernetes, drbd/nbd based
persistent volumes which can disconnect under load and
result in the situations with disks return I/O errors.

This commit also simplifies listing operation, listing
never returns any error. We can avoid this since we pretty
much ignore most of the errors anyways. When objects are
accessed directly we return proper errors.

cmd/xl-v1.go

@@ -49,9 +49,6 @@ type xlObjects struct {
 	listPool *treeWalkPool
 }
 
-// list of all errors that can be ignored in tree walk operation in XL
-var xlTreeWalkIgnoredErrs = append(baseIgnoredErrs, errDiskAccessDenied, errVolumeNotFound, errFileNotFound)
-
 // Shutdown function for object storage interface.
 func (xl xlObjects) Shutdown(ctx context.Context) error {
 	// Add any object layer shutdown activities here.