smbfs: add bdi backing to mount session

This ensures that dirty data gets flushed properly.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/include/linux/smb_fs_sb.h b/include/linux/smb_fs_sb.h
index 8a060a7..bb947dd 100644
--- a/include/linux/smb_fs_sb.h
+++ b/include/linux/smb_fs_sb.h
@@ -10,6 +10,7 @@
 #define _SMB_FS_SB
 
 #include <linux/types.h>
+#include <linux/backing-dev.h>
 #include <linux/smb.h>
 
 /*
@@ -74,6 +75,8 @@
 	struct smb_ops *ops;
 
 	struct super_block *super_block;
+
+	struct backing_dev_info bdi;
 };
 
 static inline int