uapi: fix statx attribute value overlap for DAX & MOUNT_ROOT

STATX_ATTR_MOUNT_ROOT and STATX_ATTR_DAX got merged with the same value,
so one of them needs fixing.  Move STATX_ATTR_DAX.

While we're in here, clarify the value-matching scheme for some of the
attributes, and explain why the value for DAX does not match.

Fixes: 80340fe3 ("statx: add mount_root")
Fixes: 712b2698 ("fs/stat: Define DAX statx attribute")
Link: https://lore.kernel.org/linux-fsdevel/7027520f-7c79-087e-1d00-743bdefa1a1e@redhat.com/
Link: https://lore.kernel.org/lkml/20201202214629.1563760-1-ira.weiny@intel.com/


Reported-by: David Howells <dhowells@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: David Howells <dhowells@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: <stable@vger.kernel.org> # 5.8
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>