dax: remove __generic_fsdax_supported

Just implement generic_fsdax_supported directly out of line instead of
adding a wrapper.  Given that generic_fsdax_supported is only supplied
for CONFIG_FS_DAX builds this also allows to not provide it at all for
!CONFIG_FS_DAX builds.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/20210826135510.6293-7-hch@lst.de


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