decouple DAX from block devices v2

[Christoph Hellwig]

Hi Dan,

this series decouples the DAX from the block layer so that the
block_device is not needed at all for the DAX I/O path.

Changes since v1:
 - rebase on latest v5.16-rc
 - ensure the new dax zeroing helpers are always declared
 - fix a dax_dev leak in pmem_attach_disk
 - remove '\n' from an xfs format string
 - fix a pre-existing error handling bug in alloc_dev
 - fix a few whitespace issues
 - tighten an error check
 - use s64/u64 a little more
 - improve a few commit messages
 - add a CONFIG_FS_DAX ifdef to stub out IOMAP_DAX
 - improve how IOMAP_DAX is introduced and better document why it is
   added