vsock: Move iter outside of the loop in process_rx_queue

the iter() function is used for produce a queue iterator to iterate over
the descriptors.

But usually, it shouldn't be in the while loop, which might brings more
unnecessary overhead.

So move `iter` outside of the while loop.

And the process_tx_queue has the same problem, maybe we can fix it, too.

Signed-off-by: Li Zebin <[email protected]>

Author: cutelizebin

crates/vsock/src/vhu_vsock_thread.rs

@@ -453,11 +453,11 @@ impl VhostUserVsockThread {
 
         let queue = vring_mut.get_queue_mut();
 
-        while let Some(mut avail_desc) = queue
+        let mut queue_iter = queue
             .iter(atomic_mem.memory())
-            .map_err(|_| Error::IterateQueue)?
-            .next()
-        {
+            .map_err(|_| Error::IterateQueue)?;
+
+        for mut avail_desc in queue_iter {
             used_any = true;
             let mem = atomic_mem.clone().memory();