rust-agent: Remove 'mut' where not needed

Addresses the following warning (and a few similar ones):
    warning: variable does not need to be mutable
       --> rustjail/src/container.rs:369:9
        |
    369 |     let mut oci_process: oci::Process = serde_json::from_str(process_str)?;
        |         ----^^^^^^^^^^^
        |         |
        |         help: remove this `mut`
        |
        = note: `#[warn(unused_mut)]` on by default

Fixes: #750

Signed-off-by: Christophe de Dinechin <dinechin@redhat.com>

src/agent/rustjail/src/cgroups/fs/mod.rs

@@ -468,7 +468,7 @@ fn build_blk_io_device_throttle_resource(
 fn linux_device_to_cgroup_device(d: &LinuxDevice) -> DeviceResource {
     let dev_type = DeviceType::from_char(d.r#type.chars().next()).unwrap();
 
-    let mut permissions = vec![
+    let permissions = vec![
         DevicePermissions::Read,
         DevicePermissions::Write,
         DevicePermissions::MkNod,
@@ -518,7 +518,7 @@ fn lines_to_map(content: &str) -> HashMap<String, u64> {
         .lines()
         .map(|x| x.split_whitespace().collect::<Vec<&str>>())
         .filter(|x| x.len() == 2 && x[1].parse::<u64>().is_ok())
-        .fold(HashMap::new(), |mut hm, mut x| {
+        .fold(HashMap::new(), |mut hm, x| {
             hm.insert(x[0].to_string(), x[1].parse::<u64>().unwrap());
             hm
         })

src/agent/rustjail/src/container.rs

@@ -364,7 +364,7 @@ fn do_init_child(cwfd: RawFd) -> Result<()> {
 
     let buf = read_sync(crfd)?;
     let process_str = std::str::from_utf8(&buf)?;
-    let mut oci_process: oci::Process = serde_json::from_str(process_str)?;
+    let oci_process: oci::Process = serde_json::from_str(process_str)?;
     log_child!(cfd_log, "notify parent to send cgroup manager");
     write_sync(cwfd, SYNC_SUCCESS, "")?;
 
@@ -799,9 +799,9 @@ impl BaseContainer for LinuxContainer {
             unistd::close(pwfd);
         });
 
-        let mut child_stdin: std::process::Stdio;
-        let mut child_stdout: std::process::Stdio;
-        let mut child_stderr: std::process::Stdio;
+        let child_stdin: std::process::Stdio;
+        let child_stdout: std::process::Stdio;
+        let child_stderr: std::process::Stdio;
 
         if tty {
             let pseudo = pty::openpty(None, None)?;
@@ -865,7 +865,7 @@ impl BaseContainer for LinuxContainer {
             child = child.env(FIFO_FD, format!("{}", fifofd));
         }
 
-        let mut child = child.spawn()?;
+        let child = child.spawn()?;
 
         unistd::close(crfd)?;
         unistd::close(cwfd)?;