dm vdo wait-queue: rename to vdo_waitq_dequeue_waiter

Rename vdo_waitq_dequeue_next_waiter to vdo_waitq_dequeue_waiter. The
"next" aspect of returned waiter is implied. "next" also isn't
informative ("oldest" would be). Removing "next_" adds symmetry to
vdo_waitq_enqueue_waiter().

Also fix whitespace and comments from previous waitq commit.

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

dm vdo wait-queue: remove unused debug function vdo_waitq_get_next_waiter

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

dm vdo wait-queue: add proper namespace to interface

Rename various interfaces and structs associated with vdo's wait-queue,
e.g.: s/wait_queue/vdo_wait_queue/, s/waiter/vdo_waiter/, etc.

Now all function names start with "vdo_waitq_" or "vdo_waiter_".

Reviewed-by: Ken Raeburn <raeburn@redhat.com>
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>