VFS: Remove writeback_control->fs_private

The only user of this field was NFS.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

include/linux/writeback.h

@@ -61,8 +61,6 @@ struct writeback_control {
 	unsigned for_reclaim:1;		/* Invoked from the page allocator */
 	unsigned for_writepages:1;	/* This is a writepages() call */
 	unsigned range_cyclic:1;	/* range_start is cyclic */
-
-	void *fs_private;		/* For use by ->writepages() */
 };
 
 /*