fsdax: don't require CONFIG_BLOCK

The file system DAX code now does not require the block code.  So allow
building a kernel with fuse DAX but not block layer.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20211129102203.2243509-30-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>