ext4: let the block device know when unused blocks can be discarded

Let the block device know when unused blocks can be discarded, using
the new sb_issue_discard() interface.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/mballoc.h b/fs/ext4/mballoc.h
index 9e815c4..94cb7b9 100644
--- a/fs/ext4/mballoc.h
+++ b/fs/ext4/mballoc.h
@@ -18,6 +18,8 @@
 #include <linux/pagemap.h>
 #include <linux/seq_file.h>
 #include <linux/version.h>
+#include <linux/blkdev.h>
+#include <linux/marker.h>
 #include "ext4_jbd2.h"
 #include "ext4.h"
 #include "group.h"