ecryptfs: 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/fs/ecryptfs/ecryptfs_kernel.h b/fs/ecryptfs/ecryptfs_kernel.h
index bc71154..bfc2e0f 100644
--- a/fs/ecryptfs/ecryptfs_kernel.h
+++ b/fs/ecryptfs/ecryptfs_kernel.h
@@ -35,6 +35,7 @@
 #include <linux/scatterlist.h>
 #include <linux/hash.h>
 #include <linux/nsproxy.h>
+#include <linux/backing-dev.h>
 
 /* Version verification for shared data structures w/ userspace */
 #define ECRYPTFS_VERSION_MAJOR 0x00
@@ -393,6 +394,7 @@
 struct ecryptfs_sb_info {
 	struct super_block *wsi_sb;
 	struct ecryptfs_mount_crypt_stat mount_crypt_stat;
+	struct backing_dev_info bdi;
 };
 
 /* file private data. */