md: rename "mdk_personality" to "md_personality"

"mdk" doesn't mean anything any more.

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
index 9650593..d32c785 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -523,7 +523,7 @@
 	return 0;
 }
 
-static struct mdk_personality multipath_personality =
+static struct md_personality multipath_personality =
 {
 	.name		= "multipath",
 	.level		= LEVEL_MULTIPATH,