binder: fix async_free_space accounting for empty parcels

In the Linux kernel, the following vulnerability has been resolved:

binder: fix async_free_space accounting for empty parcels

In 4.13, commit 74310e06be4d ("android: binder: Move buffer out of area shared with user space")

fixed a kernel structure visibility issue. As part of that patch,

sizeof(void *) was used as the buffer size for 0-length data payloads so

the driver could detect abusive clients sending 0-length asynchronous

transactions to a server by enforcing limits on async_free_size.

Unfortunately, on the "free" side, the accounting of async_free_space

did not add the sizeof(void *) back. The result was that up to 8-bytes of

async_free_space were leaked on every async transaction of 8-bytes or

less. These small transactions are uncommon, so this accounting issue

has gone undetected for several years.

The fix is to use "buffer_size" (the allocated buffer size) instead of

"size" (the logical buffer size) when updating the async_free_space

during the free operation. These are the same except for this

corner case of asynchronous transactions with payloads < 8 bytes.