agent/rustjail: Clean up error path in execute_hook()s async task

Clippy (in Rust 1.51 at least) has some complaints about this closure
inside execute_hook() because it uses explicit returns in some places
where it doesn't need them, because they're the last expression in the
function.

That isn't necessarily obvious from a glance, but we can make clippy happy
and also make things a little clearer: first we replace a somewhat verbose
'match' using Option::ok_or_else(), then rearrange the remaining code to
put all the error path first with an explicit return then the "happy" path
as the stright line exit with an implicit return.

fixes #1611

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

src/agent/rustjail/src/container.rs

@@ -1528,28 +1528,23 @@ async fn execute_hook(logger: &Logger, h: &Hook, st: &OCIState) -> Result<()> {
 
         match child.wait().await {
             Ok(exit) => {
-                let code = match exit.code() {
-                    Some(c) => c,
-                    None => {
-                        return Err(anyhow!("hook exit status has no status code"));
-                    }
-                };
+                let code = exit
+                    .code()
+                    .ok_or_else(|| anyhow!("hook exit status has no status code"))?;
 
-                if code == 0 {
-                    debug!(logger, "hook {} exit status is 0", &path);
-                    return Ok(());
-                } else {
+                if code != 0 {
                     error!(logger, "hook {} exit status is {}", &path, code);
                     return Err(anyhow!(nix::Error::from_errno(Errno::UnknownErrno)));
                 }
+
+                debug!(logger, "hook {} exit status is 0", &path);
+                Ok(())
             }
-            Err(e) => {
-                return Err(anyhow!(
-                    "wait child error: {} {}",
-                    e,
-                    e.raw_os_error().unwrap()
-                ));
-            }
+            Err(e) => Err(anyhow!(
+                "wait child error: {} {}",
+                e,
+                e.raw_os_error().unwrap()
+            )),
         }
     });