dm: introduce enum dm_queue_mode to cleanup related code

Introduce an enumeration type for the queue mode.  This patch does
not change any functionality but makes the DM code easier to read.

Signed-off-by: Bart Van Assche <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Signed-off-by: Mike Snitzer <[email protected]>
7 files changed