MFC r314549: Execute last ZIO of log commit synchronously.

For short transactions overhead of context switch can be too large.
Skipping it gives significant latency reduction. For large ones,
including multiple ZIOs, latency is less critical, while throughput
there may become limited by checksumming speed of single CPU core.
To get best of both cases, execute last ZIO directly from calling
thread context to save latency, while all others (if there are any)
enqueue to taskqueues in traditional way.