dm vdo: add slab structure, slab journal and reference counters

Most of the vdo volume belongs to the slab depot. The depot contains a
collection of slabs. The slabs can be up to 32GB, and are divided into
three sections. Most of a slab consists of a linear sequence of 4K blocks.
These blocks are used either to store data, or to hold portions of the
block map (see subsequent patches). In addition to the data blocks, each
slab has a set of reference counters, using 1 byte for each data block.
Finally each slab has a journal. Reference updates are written to the slab
journal, which is written out one block at a time as each block fills. A
copy of the reference counters is kept in memory, and are written out a
block at a time, in oldest-dirtied-order whenever there is a need to
reclaim slab journal space. The journal is used both to ensure that the
main recovery journal (see subsequent patches) can regularly free up space,
and also to amortize the cost of updating individual reference blocks.

This patch adds the slab structure as well as the slab journal and
reference counters.

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: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>