md: add MODULE_DESCRIPTION for all md related modules.

Suggested by  Oren Held <orenhe@il.ibm.com>

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid6algos.c b/drivers/md/raid6algos.c
index 8ce102c..bffc61b 100644
--- a/drivers/md/raid6algos.c
+++ b/drivers/md/raid6algos.c
@@ -150,3 +150,4 @@
 subsys_initcall(raid6_select_algo);
 module_exit(raid6_exit);
 MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("RAID6 Q-syndrome calculations");