dm: fix missed error code if .end_io isn't implemented by target_type

In bio-based DM's clone_endio(), when target_type doesn't implement
.end_io (e.g. linear) r will be always be initialized 0.  So if a
WRITE SAME bio fails WRITE SAME will not be disabled as intended.

Fix this by initializing r to error, rather than 0, in clone_endio().

Signed-off-by: Alex Chen <alex.chen@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Fixes: 7eee4ae2 ("dm: disable WRITE SAME if it fails")
Cc: stable@vger.kernel.org