Fix the mempool rate limiter.

Before, btcd was rate limiting all transactions that had a minimum
fee of zero.  Now, btcd only rate limits transactions that contain
a fee less than the calculated fee based on size.
Instead of declaring this here, since it's not needed for the if just below, I'd calculate it below the first 'return 0', and just make this function 'return calcMinTxRelayFee(tx)'.

No reason to calculate something if it's not going to be used.