FROMLIST: binder: protect binder_ref with outer lock

(from https://patchwork.kernel.org/patch/9817771/

)

Use proc->outer_lock to protect the binder_ref structure.
The outer lock allows functions operating on the binder_ref
to do nested acquires of node and inner locks as necessary
to attach refs to nodes atomically.

Binder refs must never be accesssed without holding the
outer lock.

Change-Id: Iffb9ae47fd383b87b70ee6bec344cde9f8d24996
Signed-off-by: Todd Kjos <tkjos@google.com>