rust_binder: Avoid holding lock when dropping delivered_death

In 6c37bebd8c, we switched to looping over the list and dropping each
individual node, ostensibly without the lock held in the loop body.

If the kernel were using Rust Edition 2024, the comment would be
accurate, and the lock would not be held across the drop. However, the
kernel is currently using 2021, so tail expression lifetime extension
results in the lock being held across the drop. Explicitly binding the
expression result to a variable makes the lockguard no longer part of a
tail expression, causing the lock to be dropped before entering the loop
body.

This was detected via `CONFIG_PROVE_LOCKING` identifying an invalid wait
context at the drop site.

Reported-by: David Stevens <stevensd@google.com>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Cc: stable <stable@kernel.org>
Fixes: 6c37bebd8c ("rust_binder: avoid mem::take on delivered_deaths")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Acked-by: Carlos Llamas <cmllamas@google.com>
Link: https://patch.msgid.link/20260403-lockhold-v1-1-c332b56cd8ae@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/android/binder/process.rs

@@ -1402,7 +1402,12 @@ fn deferred_release(self: Arc<Self>) {
         // Clear delivered_deaths list.
         //
         // Scope ensures that MutexGuard is dropped while executing the body.
-        while let Some(delivered_death) = { self.inner.lock().delivered_deaths.pop_front() } {
+        while let Some(delivered_death) = {
+            // Explicitly bind to avoid tail expression lifetime extension of the lockguard
+            // Can be removed when the kernel moves to edition 2024
+            let maybe_death = self.inner.lock().delivered_deaths.pop_front();
+            maybe_death
+        } {
             drop(delivered_death);
         }