rust_binder: move BC_FREE_BUFFER drop inside if statement

When looking at flamegraphs, there is a pretty large entry for the
function call drop_in_place::<Option<Allocation>> which in turn calls
drop_in_place::<Allocation>. Combined with the looper_need_return
condition, this means that the generated code looks like this:

if let Some(buffer) = buffer {
if buffer.looper_need_return_on_free() {
self.inner.lock().looper_need_return = true;
}
}
drop_in_place::<Option<Allocation>>() { // not inlined
if let Some(buffer) = buffer {
drop_in_place::<Allocation>(buffer);
}
}

This kind of situation where you check X and then check X again is
normally optimized into a single condition, but in this case due to the
non-inlined function call to drop_in_place::<Option<Allocation>>, that
optimization does not happen.

Furthermore, the drop_in_place::<Allocation> call is only two-thirds of
the drop_in_place::<Option<Allocation>> call in the flamegraph. This
indicates that this double condition is not performing well. Also, last
time I looked at Binder perf, I remember finding that the destructor of
Allocation was involved with many branch mispredictions.

Thus, change this code to look like this:

if let Some(buffer) = buffer {
if buffer.looper_need_return_on_free() {
self.inner.lock().looper_need_return = true;
}
drop_in_place::<Allocation>(buffer);
}

by dropping the Allocation directly. Flamegraphs confirm that the
drop_in_place::<Option<Allocation>> call disappears from this change.

Signed-off-by: Alice Ryhl <[e-mail]>
Acked-by: Carlos Llamas <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/android/binder/thread.rs

···
                 }
                 BC_FREE_BUFFER => {
                     let buffer = self.process.buffer_get(reader.read()?);
-                    if let Some(buffer) = &buffer {
+                    if let Some(buffer) = buffer {
                         if buffer.looper_need_return_on_free() {
                             self.inner.lock().looper_need_return = true;
                         }
+                        drop(buffer);
                     }
-                    drop(buffer);
                 }
                 BC_INCREFS => {
                     self.process