A fix has now been produced, for the dax compile fail with 5.9-rc6:

Reported:
http://lkml.iu.edu/hypermail/linux/kernel/2009.2/07060.html

Fix:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=88b67edd7247466bc47f01e1dc539b0d0d4b931e

I have tested this, and the kernel now compiles OK, and loads/runs as expected..

Robert Gadsdon.   September 21st 2020.