stop using buffer heads in xfs and iomap

[Christoph Hellwig <hch@xxxxxx>]

Hi all,

this series adds support for reading blocks from disk using the iomap
interface, and then gradually switched the buffered I/O path to not
require buffer heads.  It has survived xfstests for 1k and 4k block
size.

There are various small changes to the core VFS, block and readahead
code to make this happen.


A git tree is available at:

    git://git.infradead.org/users/hch/xfs.git xfs-remove-bufferheads

Gitweb:

    http://git.infradead.org/users/hch/xfs.git/shortlog/refs/heads/xfs-remove-bufferheads