dm mpath: add optional "queue_mode" feature

Allow a user to specify an optional feature 'queue_mode <mode>' where
<mode> may be "bio", "rq" or "mq" -- which corresponds to bio-based,
request_fn rq-based, and blk-mq rq-based respectively.

If the queue_mode feature isn't specified the default for the
"multipath" target is still "rq" but if dm_mod.use_blk_mq is set to Y
it'll default to mode "mq".

This new queue_mode feature introduces the ability for each multipath
device to have its own queue_mode (whereas before this feature all
multipath devices effectively had to have the same queue_mode).

This commit also goes a long way to eliminate the awkward (ab)use of
DM_TYPE_*, the associated filter_md_type() and other relatively fragile
and difficult to maintain code.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

include/linux/device-mapper.h

@@ -19,6 +19,14 @@ struct dm_table;
 struct mapped_device;
 struct bio_vec;
 
+/*
+ * Type of table, mapped_device's mempool and request_queue
+ */
+#define DM_TYPE_NONE			0
+#define DM_TYPE_BIO_BASED		1
+#define DM_TYPE_REQUEST_BASED		2
+#define DM_TYPE_MQ_REQUEST_BASED	3
+
 typedef enum { STATUSTYPE_INFO, STATUSTYPE_TABLE } status_type_t;
 
 union map_info {
@@ -443,6 +451,14 @@ int dm_table_add_target(struct dm_table *t, const char *type,
  */
 void dm_table_add_target_callbacks(struct dm_table *t, struct dm_target_callbacks *cb);
 
+/*
+ * Target can use this to set the table's type.
+ * Can only ever be called from a target's ctr.
+ * Useful for "hybrid" target (supports both bio-based
+ * and request-based).
+ */
+void dm_table_set_type(struct dm_table *t, unsigned type);
+
 /*
  * Finally call this to make the table ready for use.
  */