FROMLIST: binder: protect against two threads freeing buffer

(from https://patchwork.kernel.org/patch/9817815/

)

Adds protection against malicious user code freeing
the same buffer at the same time which could cause
a crash. Cannot happen under normal use.

Bug: 36650912
Change-Id: I43e078cbf31c0789aaff5ceaf8f1a94c75f79d45
Test: tested manually
Signed-off-by: Todd Kjos <tkjos@google.com>