dm vdo indexer delta-index: fix typos in comments

Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

dm vdo: move indexer files into sub-directory

The goal is to assist high-level understanding of which code is
conceptually specific to VDO's indexer.

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Matthew Sakai <msakai@redhat.com>