Add a transaction ID to track the first element in the transaction
array that has been polled.  Advance the ID when we are releasing a
transaction.

Temporarily add warnings that verify that the first polled
transaction tracked by the ID matches the first element on the
polled list, both when polling and freeing.

Remove the temporary warnings added by the previous commit.
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>