rpk: bugfix - filter in list-mount

We should default to filter `all`, otherwise we
won't print any operation unless a filter is
provided.

src/go/rpk/pkg/cli/cluster/storage/cancel-mount.go

@@ -65,7 +65,7 @@ Cancel a mount/unmount operation
 				out.MaybeDie(err, "unable to cancel the mount/unmount operation: %v", err)
 			}
 
-			fmt.Printf("Successfully canceled the operation with ID %v", migrationID)
+			fmt.Printf("Successfully canceled the operation with ID %v\n", migrationID)
 		},
 	}
 	return cmd

src/go/rpk/pkg/cli/cluster/storage/list-mount.go

@@ -80,7 +80,7 @@ Use filter to list only migrations in a specific state
 		},
 	}
 	p.InstallFormatFlag(cmd)
-	cmd.Flags().StringVarP(&filter, "filter", "f", "", "Filter the list of migrations by state. Only valid for text")
+	cmd.Flags().StringVarP(&filter, "filter", "f", "all", "Filter the list of migrations by state. Only valid for text")
 	return cmd
 }