binder: make sure target_node has strong ref

When initiating a transaction, the target_node must
have a strong ref on it. Then we take a second
strong ref to make sure the node survives until the
transaction is complete.

Signed-off-by: Todd Kjos <tkjos@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>