dax: fix up some of the block device related ifdefs

The DAX device <-> block device association is only enabled if
CONFIG_BLOCK is enabled.  Update dax.h to account for that and use
the right conditions for the fs_put_dax stub as well.

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/20211129102203.2243509-28-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>