NFS: Fall back to synchronous writes when a background write errors...

This helps prevent huge queues of background writes from building up
whenever the server runs out of disk or quota space, or if someone changes
the file access modes behind our backs.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

include/linux/nfs_fs.h

@@ -77,6 +77,9 @@ struct nfs_open_context {
 	struct nfs4_state *state;
 	fl_owner_t lockowner;
 	int mode;
+
+	unsigned long flags;
+#define NFS_CONTEXT_ERROR_WRITE		(0)
 	int error;
 
 	struct list_head list;