NFS: Fix a umount race

Ensure that we unregister the bdi before kill_anon_super() calls
ida_remove() on our device name.

Signed-off-by: Trond Myklebust <[e-mail]>
Cc: [e-mail]

fs/nfs/super.c

···
 static int nfs_get_sb(struct file_system_type *, int, const char *, void *, struct vfsmount *);
 static int nfs_xdev_get_sb(struct file_system_type *fs_type,
 		int flags, const char *dev_name, void *raw_data, struct vfsmount *mnt);
+static void nfs_put_super(struct super_block *);
 static void nfs_kill_super(struct super_block *);
 static int nfs_remount(struct super_block *sb, int *flags, char *raw_data);
 
···
 	.alloc_inode	= nfs_alloc_inode,
 	.destroy_inode	= nfs_destroy_inode,
 	.write_inode	= nfs_write_inode,
+	.put_super	= nfs_put_super,
 	.statfs		= nfs_statfs,
 	.clear_inode	= nfs_clear_inode,
 	.umount_begin	= nfs_umount_begin,
···
 	.alloc_inode	= nfs_alloc_inode,
 	.destroy_inode	= nfs_destroy_inode,
 	.write_inode	= nfs_write_inode,
+	.put_super	= nfs_put_super,
 	.statfs		= nfs_statfs,
 	.clear_inode	= nfs4_clear_inode,
 	.umount_begin	= nfs_umount_begin,
···
 }
 
 /*
+ * Ensure that we unregister the bdi before kill_anon_super
+ * releases the device name
+ */
+static void nfs_put_super(struct super_block *s)
+{
+	struct nfs_server *server = NFS_SB(s);
+
+	bdi_unregister(&server->backing_dev_info);
+}
+
+/*
  * Destroy an NFS2/3 superblock
  */
 static void nfs_kill_super(struct super_block *s)
···
 	struct nfs_server *server = NFS_SB(s);
 
 	kill_anon_super(s);
-	bdi_unregister(&server->backing_dev_info);
 	nfs_fscache_release_super_cookie(s);
 	nfs_free_server(server);
 }