df: fix panic when using -x argument (#3257)

Also allow multiple '-x' options

src/uu/df/src/df.rs

@@ -442,8 +442,10 @@ pub fn uu_app<'a>() -> App<'a> {
             Arg::new(OPT_EXCLUDE_TYPE)
                 .short('x')
                 .long("exclude-type")
+                .allow_invalid_utf8(true)
                 .takes_value(true)
                 .use_delimiter(true)
+                .multiple_occurrences(true)
                 .help("limit listing to file systems not of type TYPE"),
         )
         .arg(Arg::new(OPT_PATHS).multiple_occurrences(true))

tests/by-util/test_df.rs

@@ -85,6 +85,11 @@ fn test_type_option() {
     new_ucmd!().args(&["-t", "ext4", "-t", "ext3"]).succeeds();
 }
 
+#[test]
+fn test_exclude_type_option() {
+    new_ucmd!().args(&["-x", "ext4", "-x", "ext3"]).succeeds();
+}
+
 #[test]
 fn test_total() {
     // Example output: