dm vdo: add deduplication index storage interface

This patch adds infrastructure for managing reads and writes to the
underlying storage layer for the deduplication index. The deduplication
index uses dm-bufio for all of its reads and writes, so part of this
infrastructure is managing the various dm-bufio clients required. It also
adds the buffered reader and buffered writer abstractions, which simplify
reading and writing metadata structures that span several blocks.

This patch also includes structures and utilities for encoding and decoding
all of the deduplication index metadata, collectively called the index
layout.

Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
Co-developed-by: Michael Sclafani <dm-devel@lists.linux.dev>
Signed-off-by: Michael Sclafani <dm-devel@lists.linux.dev>
Co-developed-by: Thomas Jaskiewicz <tom@jaskiewicz.us>
Signed-off-by: Thomas Jaskiewicz <tom@jaskiewicz.us>
Co-developed-by: John Wiele <jwiele@redhat.com>
Signed-off-by: John Wiele <jwiele@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>