Make AIO to not allocate pbufs for unmapped I/O like r281825.

While there, make few more performance optimizations.

On 40-core system doing many 512-byte AIO reads from array of raw SSDs
this change removes lock congestions inside pbuf allocator and devfs,
and bottleneck on single AIO completion taskqueue thread. It improves
peak AIO performance from ~600K to ~1.3M IOPS.

MFC after: 2 weeks