binder: add spinlock to protect binder_node

node->node_lock is used to protect elements of node. No
need to acquire for fields that are invariant: debug_id,
ptr, cookie.
Signed-off-by: NTodd Kjos <tkjos@google.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>