iomap infrastructure and multipage writes V5

[Christoph Hellwig]

On Tue, Jun 28, 2016 at 10:26:49AM +1000, Dave Chinner wrote:
> Christoph, it look slike there's an ENOSPC+ENOMEM behavioural regression here.
> generic/224 on my 1p/1GB RAM VM using a 1k lock size filesystem has
> significantly different behaviour once ENOSPC is hit withi this patchset.

Works fine on my 1k test setup with 4 CPUs and 2GB RAM.  1 CPU and 1GB
RAM runs into the OOM killer, although I haven't checked if that was
the case with the old code as well.  I'll look into this more later
today or tomorrow.