Hi all,

this is a slight rework of the patches from Andreas to prepare for gfs2
using the iomap code.

Note: I'd like to start with an immutable branch for iomap patches in either
the XFS tree or a tree of mine at the beginning of the merge window so that
we have a common base for the GFS2 and XFS iomap work.

Changes since v1:
 - move code to a different spot in iomap.c to prefer for readpages
   inline data support
 - add a forward declaration for struct page to iomap.h
 - fix a typo in the dax_dev/bdev unioning
 - fix a comment typo