dm raid: add missing cleanup in raid_ctr()

If rs_prepare_reshape() fails, no cleanup is executed, leading to
leak of the raid_set structure allocated at the beginning of
raid_ctr(). To fix this issue, go to the label 'bad' if the error
occurs.

Fixes: 11e47232 ("dm raid: stop keeping raid set frozen altogether")
Cc: stable@vger.kernel.org
Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>