dax: fix false CONFIG_BLOCK dependency

In the BLOCK=n case the dax core does not need to / must not emit the
block-device-dax helpers. Otherwise it leads to compile errors.

Cc: Arnd Bergmann <arnd@arndb.de>
Reported-by: Fabian Frederick <fabf@skynet.be>
Fixes: ef51042472f5 ("block, dax: move 'select DAX' from BLOCK to FS_DAX")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dax/super.c

@@ -44,6 +44,7 @@ void dax_read_unlock(int id)
 }
 EXPORT_SYMBOL_GPL(dax_read_unlock);
 
+#ifdef CONFIG_BLOCK
 int bdev_dax_pgoff(struct block_device *bdev, sector_t sector, size_t size,
 		pgoff_t *pgoff)
 {
@@ -112,6 +113,7 @@ int __bdev_dax_supported(struct super_block *sb, int blocksize)
 	return 0;
 }
 EXPORT_SYMBOL_GPL(__bdev_dax_supported);
+#endif
 
 /**
  * struct dax_device - anchor object for dax services