dm: move request-based code out to dm-rq.[hc]

Add some seperation between bio-based and request-based DM core code.

'struct mapped_device' and other DM core only structures and functions
have been moved to dm-core.h and all relevant DM core .c files have been
updated to include dm-core.h rather than dm.h

DM targets should _never_ include dm-core.h!

[block core merge conflict resolution from Stephen Rothwell]
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
diff --git a/drivers/md/dm-stats.c b/drivers/md/dm-stats.c
index 4fba26c..38b05f2 100644
--- a/drivers/md/dm-stats.c
+++ b/drivers/md/dm-stats.c
@@ -10,7 +10,7 @@
 #include <linux/module.h>
 #include <linux/device-mapper.h>
 
-#include "dm.h"
+#include "dm-core.h"
 #include "dm-stats.h"
 
 #define DM_MSG_PREFIX "stats"