dm, dax: Make sure dm_dax_flush() is called if device supports it

Currently dm_dax_flush() is not being called, even if underlying dax
device supports write cache, because DAXDEV_WRITE_CACHE is not being
propagated up to the DM dax device.

If the underlying dax device supports write cache, set
DAXDEV_WRITE_CACHE on the DM dax device.  This will cause dm_dax_flush()
to be called.

Fixes: abebfbe2f7 ("dm: add ->flush() dax operation support")
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
diff --git a/drivers/dax/super.c b/drivers/dax/super.c
index ce9e563..938eb48 100644
--- a/drivers/dax/super.c
+++ b/drivers/dax/super.c
@@ -278,6 +278,12 @@ void dax_write_cache(struct dax_device *dax_dev, bool wc)
 }
 EXPORT_SYMBOL_GPL(dax_write_cache);
 
+bool dax_write_cache_enabled(struct dax_device *dax_dev)
+{
+	return test_bit(DAXDEV_WRITE_CACHE, &dax_dev->flags);
+}
+EXPORT_SYMBOL_GPL(dax_write_cache_enabled);
+
 bool dax_alive(struct dax_device *dax_dev)
 {
 	lockdep_assert_held(&dax_srcu);