dm vdo data-vio: rename is_trim flag to is_discard

Eliminate use of "trim" in favor of "discard" since it reflects the
top-level Linux discard primative rather than the ATA specific ditto.

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

dm vdo: add vdo type declarations, constants, and simple data structures

Add definitions of constants defining the fixed parameters of a VDO
volume, and the default and maximum values of configurable or dynamic
parameters.

Add definitions of internal status codes used for internal
communication within the module and for logging.

Add definitions of types and structs used to manage the processing of
an I/O operation.

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