nfs: fix race in nfs_dirty_request

When called from nfs_flush_incompatible, the req is not locked, so
req->wb_page might be set to NULL before it is used by PageWriteback.
Signed-off-by: NFred Isaman <iisaman@citi.umich.edu>
Signed-off-by: NBenny Halevy <bhalevy@panasas.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/write.c

@@ -415,7 +415,7 @@ nfs_dirty_request(struct nfs_page *req)
 
 	if (page == NULL || test_bit(PG_NEED_COMMIT, &req->wb_flags))
 		return 0;
-	return !PageWriteback(req->wb_page);
+	return !PageWriteback(page);
 }
 
 #if defined(CONFIG_NFS_V3) || defined(CONFIG_NFS_V4)