iomap: build the block based code conditionally

Only build the block based iomap code if CONFIG_BLOCK is set.  Currently
that is always the case, but it will change soon.

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/20211129102203.2243509-29-hch@lst.de
Signed-off-by: Dan Williams <dan.j.williams@intel.com>