dax/super: Do not request a pointer kaddr when not required

Function __bdev_dax_supported doesn't need to get local pointer kaddr
from direct_access. Using NULL instead of having to pass in a useless
local pointer that caller then just throw away.
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

drivers/dax/super.c

@@ -88,7 +88,6 @@ bool __bdev_dax_supported(struct block_device *bdev, int blocksize)
 	bool dax_enabled = false;
 	pgoff_t pgoff;
 	int err, id;
-	void *kaddr;
 	pfn_t pfn;
 	long len;
 	char buf[BDEVNAME_SIZE];
@@ -114,7 +113,7 @@ bool __bdev_dax_supported(struct block_device *bdev, int blocksize)
 	}
 
 	id = dax_read_lock();
-	len = dax_direct_access(dax_dev, pgoff, 1, &kaddr, &pfn);
+	len = dax_direct_access(dax_dev, pgoff, 1, NULL, &pfn);
 	dax_read_unlock(id);
 
 	put_dax(dax_dev);