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/linear.c b/drivers/md/linear.c
index 3b3f77c..00435bd 100644
--- a/drivers/md/linear.c
+++ b/drivers/md/linear.c
@@ -383,6 +383,7 @@
 module_init(linear_init);
 module_exit(linear_exit);
 MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("Linear device concatenation personality for MD");
 MODULE_ALIAS("md-personality-1"); /* LINEAR - deprecated*/
 MODULE_ALIAS("md-linear");
 MODULE_ALIAS("md-level--1");