Staging: binder: Prevent the wrong thread from adding a transaction to the stack.

If a thread is part of a transaction stack, it is only allowed to make
another call if it was the target of the top transaction on the stack.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>