FROMLIST: binder: add spinlock to protect binder_node

(from https://patchwork.kernel.org/patch/9817769/

)

node->node_lock is used to protect elements of node. No
need to acquire for fields that are invariant: debug_id,
ptr, cookie.

Change-Id: I612ecb9db2d69b1319a9f0c450ccfdc85de70c39
Signed-off-by: Todd Kjos <tkjos@google.com>