dax_supported() related cleanups

Hi all,

this series first clarifies how to use fsdax in the Kconfig help a bit,
and te untangles the code path that checks if fsdax is supported.

Diffstat
 drivers/dax/super.c   |  191 +++++++++++++++++++-------------------------------
 drivers/md/dm-table.c |    9 --
 drivers/md/dm.c       |    2 
 fs/Kconfig            |   17 +++-
 fs/ext2/super.c       |    3 
 fs/ext4/super.c       |    3 
 fs/xfs/xfs_super.c    |   16 +++-
 include/linux/dax.h   |   41 +---------
 8 files changed, 113 insertions(+), 169 deletions(-)