NFSv4: Fix open(O_TRUNC) and ftruncate() error handling

If the file wasn't opened for writing, then truncate and ftruncate
need to report the appropriate errors.

Reported-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: stable@vger.kernel.org
diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 4aaf031..8789210 100644
--- a/fs/nfs/dir.c
+++ b/fs/nfs/dir.c
@@ -1429,7 +1429,7 @@
 	}
 
 	open_flags = nd->intent.open.flags;
-	attr.ia_valid = 0;
+	attr.ia_valid = ATTR_OPEN;
 
 	ctx = create_nfs_open_context(dentry, open_flags);
 	res = ERR_CAST(ctx);
@@ -1536,7 +1536,7 @@
 	if (IS_ERR(ctx))
 		goto out;
 
-	attr.ia_valid = 0;
+	attr.ia_valid = ATTR_OPEN;
 	if (openflags & O_TRUNC) {
 		attr.ia_valid |= ATTR_SIZE;
 		attr.ia_size = 0;