aio: implement request batching

Hi,

In this, the 4th iteration of the patch set, I've addressed the concerns
of Jens and Zach as follows:

- In the O_DIRECT write path, we get rid of WRITE_ODIRECT in favor of
  WRITE_SYNC_PLUG
- request batching is only done for AIO operations that could
  potentially benefit from it
- Initialization of the hash table is done using the gcc array
  initialization syntax

Further, I got rid of a compiler warning that I had introduced in the
last patch set.  I'll continue to test this under a variety of loads,
but would certainly appreciate it if others could give it a spin.

Patch overview / reason for existence:

Some workloads issue batches of small I/Os, and the performance is poor
due to the call to blk_run_address_space for every single iocb.  Nathan
Roberts pointed this out, and suggested that by deferring this call
until all I/Os in the iocb array are submitted to the block layer, we
can realize some impressive performance gains.

For example, running against a simple SATA disk driving a queue depth of
128, we can improve the performance of O_DIRECT writes twofold for 4k
I/Os, and similarly impressive numbers for other I/O sizes:
  http://people.redhat.com/jmoyer/dbras/vanilla-vs-v4/metal...

For read workloads on somewhat faster storage, we see similar benefits
for batches of smaller I/Os as well:
  http://people.redhat.com/jmoyer/dbras/vanilla-vs-v4/thor-...
That's 230+MB/s for vanilla 4k reads, batches of 16 vs. 410MB/s for
patched.  When there are multiple threads competing for the disk, I
haven't witnessed such crazy numbers, however.

Cheers,
Jeff
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