dax: stub out dax_supported for !CONFIG_FS_DAX

dax_supported calls into ->dax_supported which checks for fsdax support.
Don't bother building it for !CONFIG_FS_DAX as it will always return
false.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20210826135510.6293-8-hch@lst.de


Signed-off-by: Dan Williams <dan.j.williams@intel.com>