dm vdo indexer: fix use after free

Fixes: b46d79bd ("dm vdo: add deduplication index storage interface")
Reported-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>