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/posix-errors.go

@@ -24,22 +24,46 @@ import (
 
 // Function not implemented error
 func isSysErrNoSys(err error) bool {
-	return err == syscall.ENOSYS
+	if pathErr, ok := err.(*os.PathError); ok {
+		switch pathErr.Err {
+		case syscall.ENOSYS:
+			return true
+		}
+	}
+	return false
 }
 
 // Not supported error
 func isSysErrOpNotSupported(err error) bool {
-	return err == syscall.EOPNOTSUPP
+	if pathErr, ok := err.(*os.PathError); ok {
+		switch pathErr.Err {
+		case syscall.EOPNOTSUPP:
+			return true
+		}
+	}
+	return false
 }
 
 // No space left on device error
 func isSysErrNoSpace(err error) bool {
-	return err == syscall.ENOSPC
+	if pathErr, ok := err.(*os.PathError); ok {
+		switch pathErr.Err {
+		case syscall.ENOSPC:
+			return true
+		}
+	}
+	return false
 }
 
 // Input/output error
 func isSysErrIO(err error) bool {
-	return err == syscall.EIO
+	if pathErr, ok := err.(*os.PathError); ok {
+		switch pathErr.Err {
+		case syscall.EIO:
+			return true
+		}
+	}
+	return false
 }
 
 // Check if the given error corresponds to EISDIR (is a directory).