FROMLIST: dax: Fix compilation for CONFIG_DAX && !CONFIG_FS_DAX

dax_supported() is defined whenever CONFIG_DAX is enabled. So dummy
implementation should be defined only in !CONFIG_DAX case, not in
!CONFIG_FS_DAX case.

Fixes: e2ec5128 ("dm: Call proper helper to determine dax support")
Cc: <stable@vger.kernel.org>
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Link: https://lore.kernel.org/r/20200921095035.GC5862@quack2.suse.cz
Change-Id: I986965a9f6219e332ab3fc22a460423db58adeb1