iomap infrastructure and multipage writes V4

[Christoph Hellwig]

On Wed, Jun 01, 2016 at 04:35:53PM +1000, Dave Chinner wrote:
> I just pulled this forward to 4.7-rc1, and I get an immediate
> failure in generic/346:

> Maybe I screwed up the forward merge sorting out all the bits that
> conflicted with what went into 4.7-rc1. Perhaps it would be best if
> you rebased and reposted, Christoph?

No, I screwed up and enabled that BUG_ON for the buffered read
case accidentally - it was only supposed to be there for direct I/O.

I've rebase three and fixed it up.  I'll resend it as soon as the
xfstests run completes.