llists: move llist_reverse_order from raid5 to llist.c

Make this useful helper available for other users.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/llist.h b/include/linux/llist.h
index 8828a78..fbf10a0 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -195,4 +195,6 @@
 
 extern struct llist_node *llist_del_first(struct llist_head *head);
 
+struct llist_node *llist_reverse_order(struct llist_node *head);
+
 #endif /* LLIST_H */