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.

Change-Id: I9461229401aaa7f3b5b2477960f79d4d1bd17fee
Signed-off-by: Todd Kjos <tkjos@google.com>