dax_supported() related cleanups v2

Hi all,

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

Changes since v1:
 - improve the FS_DAX Kconfig help text further
 - write a proper commit log for a patch missing it

Diffstat
 drivers/dax/super.c   |  191 +++++++++++++++++++-------------------------------
 drivers/md/dm-table.c |    9 --
 drivers/md/dm.c       |    2 
 fs/Kconfig            |   21 ++++-
 fs/ext2/super.c       |    3 
 fs/ext4/super.c       |    3 
 fs/xfs/xfs_super.c    |   16 +++-
 include/linux/dax.h   |   41 +---------
 8 files changed, 117 insertions(+), 169 deletions(-)