blk-mq-debugfs: allow schedulers to register debugfs attributes

This provides the infrastructure for schedulers to expose their internal
state through debugfs. We add a list of queue attributes and a list of
hctx attributes to struct elevator_type and wire them up when switching
schedulers.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Hannes Reinecke <hare@suse.com>

Add missing seq_file.h header in blk-mq-debugfs.h

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index de8ed9a..c47aa24 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -60,6 +60,7 @@
 
 #ifdef CONFIG_BLK_DEBUG_FS
 	struct dentry		*debugfs_dir;
+	struct dentry		*sched_debugfs_dir;
 #endif
 };