dm vdo: add the block map

The block map contains the logical to physical mapping. It can be thought
of as an array with one entry per logical address. Each entry is 5 bytes:
36 bits contain the physical block number which holds the data for the
given logical address, and the remaining 4 bits are used to indicate the
nature of the mapping. Of the 16 possible states, one represents a logical
address which is unmapped (i.e. it has never been written, or has been
discarded), one represents an uncompressed block, and the other 14 states
are used to indicate that the mapped data is compressed, and which of the
compression slots in the compressed block this logical address maps to.

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>