binder: fix async space check for 0-sized buffers

commit 3091c21d upstream.

Move the padding of 0-sized buffers to an earlier stage to account for
this round up during the alloc->free_async_space check.

Fixes: 74310e06

 ("android: binder: Move buffer out of area shared with user space")
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Link: https://lore.kernel.org/r/20231201172212.1813387-5-cmllamas@google.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>