fix a dax/block device attribute registration regression

Hi Dan and Jens,

this series fixed a regression in how the dax/write_cache attribute of the
pmem devices was registere.  It does so by both fixing the API abuse in the
driver and (temporarily) the behavior change in the block layer that made
this API abuse not work anymore.

Diffstat:
 block/genhd.c         |    3 +-
 drivers/dax/super.c   |   64 --------------------------------------------------
 drivers/nvdimm/pmem.c |   48 ++++++++++++++++++++++++++++++++++---
 include/linux/dax.h   |    2 -
 4 files changed, 46 insertions(+), 71 deletions(-)