devices: vvu: wl: do not hold RefCell borrow across await

Change the comparison from an `if let` into a regular `if ... ==` by
implementing `PartialEq` for the relevant types. This makes the lifetime
of the `wl_state.borrow_mut()` end before calling
`kick_evt.next_val().await()`.

Fixes a new clippy warning from Rust 1.61.0:
<https://rust-lang.github.io/rust-clippy/master/index.html#await_holding_refcell_ref>

BUG=None
TEST=tools/clippy # with Rust 1.61.0

Change-Id: I067c2ab532d753306cd5334df5bb4f2c7c2e6584
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3671130
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Reviewed-by: Anton Romanov <romanton@google.com>
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>

devices/src/virtio/vhost/user/device/wl.rs

@@ -62,8 +62,8 @@ async fn run_in_queue(
             break;
         }
 
-        if let Err(wl::DescriptorsExhausted) =
-            wl::process_in_queue(&doorbell, &mut queue, &mem, &mut wlstate.borrow_mut())
+        if wl::process_in_queue(&doorbell, &mut queue, &mem, &mut wlstate.borrow_mut())
+            == Err(wl::DescriptorsExhausted)
         {
             if let Err(e) = kick_evt.next_val().await {
                 error!("Failed to read kick event for in queue: {}", e);

devices/src/virtio/wl.rs

@@ -1492,7 +1492,7 @@ impl WlState {
     }
 }
 
-#[derive(ThisError, Debug)]
+#[derive(ThisError, Debug, PartialEq)]
 #[error("no descriptors available in queue")]
 pub struct DescriptorsExhausted;