9p: 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/9p/vfs_super.c b/fs/9p/vfs_super.c
index 491108b..806da5d 100644
--- a/fs/9p/vfs_super.c
+++ b/fs/9p/vfs_super.c
@@ -77,6 +77,7 @@
 	sb->s_blocksize = 1 << sb->s_blocksize_bits;
 	sb->s_magic = V9FS_MAGIC;
 	sb->s_op = &v9fs_super_ops;
+	sb->s_bdi = &v9ses->bdi;
 
 	sb->s_flags = flags | MS_ACTIVE | MS_SYNCHRONOUS | MS_DIRSYNC |
 	    MS_NOATIME;