dax: don't pass buffer_head to dax_insert_mapping

This way we can use this helper for the iomap based DAX implementation
as well.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>