dm vdo: enable configuration and building of dm-vdo

dm-vdo targets are not supported for 32-bit configurations. A vdo target
typically requires 1 to 1.5 GB of memory at any given time, which is likely
a large fraction of the addressable memory of a 32-bit system. At the same
time, the amount of addressable storage attached to a 32-bit system may not
be large enough for deduplication to provide much benefit. Because of these
concerns, 32-bit platforms are deemed unlikely to benefit from using a vdo
target, so dm-vdo is targeted only at 64-bit platforms.

Co-developed-by: J. corwin Coburn <corwin@hurlbutnet.net>
Signed-off-by: J. corwin Coburn <corwin@hurlbutnet.net>
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>