df: always produce the same order in output table

Change the `filter_mount_list()` function so that it always produces
the same order of `MountInfo` objects. This change ultimately results
in `df` printing its table of filesystems in the same order on each
execution. Previously, the table was in an arbitrary order because the
`MountInfo` objects were read from a `HashMap`.

Fixes #3086.

tests/by-util/test_df.rs

@@ -37,4 +37,25 @@ fn test_df_output() {
     }
 }
 
+/// Test that the order of rows in the table does not change across executions.
+#[test]
+fn test_order_same() {
+    // TODO When #3057 is resolved, we should just use
+    //
+    //     new_ucmd!().arg("--output=source").succeeds().stdout_move_str();
+    //
+    // instead of parsing the entire `df` table as a string.
+    let output1 = new_ucmd!().succeeds().stdout_move_str();
+    let output2 = new_ucmd!().succeeds().stdout_move_str();
+    let output1: Vec<String> = output1
+        .lines()
+        .map(|l| String::from(l.split_once(' ').unwrap().0))
+        .collect();
+    let output2: Vec<String> = output2
+        .lines()
+        .map(|l| String::from(l.split_once(' ').unwrap().0))
+        .collect();
+    assert_eq!(output1, output2);
+}
+
 // ToDO: more tests...