Simply the TXQ_AVAIL() macro to avoid using
get_inuse(txq->ift_size, txq->ift_cidx, txq->ift_pidx, txq->ift_gen) and instead
define it in terms of txq->ift_in_use.
Note that in_use is equivalent, because it is updated at the same time
as cidx/pidx and gen, and under the same serialization. The advantage
is that unlocked reads become safer, since they get an atomic snapshot of availability,
but an unlocked read of multiple counters can produce inconsistent results.
This makes TXQ_AVAIL() cheaper due to using one load and no branches
It is also more correct because