dm raid: make raid_sets symbol static

Fixes the following sparse warning:

drivers/md/dm-raid.c:33:1: warning:
 symbol 'raid_sets' was not declared. Should it be static?
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-raid.c

@@ -30,7 +30,7 @@
 #define	MIN_RAID456_JOURNAL_SPACE (4*2048)
 
 /* Global list of all raid sets */
-LIST_HEAD(raid_sets);
+static LIST_HEAD(raid_sets);
 
 static bool devices_handle_discard_safely = false;