NFSv4 do not send an empty SETATTR compound

Commit 536e43d1 ATTR_OPEN check can result in
an ia_valid with only ATTR_FILE set, and no NFS_VALID_ATTRS attributes to
request from the server.
Signed-off-by: Andy Adamson <andros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/nfs4proc.c

@@ -2549,6 +2549,14 @@ nfs4_proc_setattr(struct dentry *dentry, struct nfs_fattr *fattr,
 
 	nfs_fattr_init(fattr);
 	
+	/* Deal with open(O_TRUNC) */
+	if (sattr->ia_valid & ATTR_OPEN)
+		sattr->ia_valid &= ~(ATTR_MTIME|ATTR_CTIME|ATTR_OPEN);
+
+	/* Optimization: if the end result is no change, don't RPC */
+	if ((sattr->ia_valid & ~(ATTR_FILE)) == 0)
+		return 0;
+
 	/* Search for an existing open(O_WRITE) file */
 	if (sattr->ia_valid & ATTR_FILE) {
 		struct nfs_open_context *ctx;
@@ -2560,10 +2568,6 @@ nfs4_proc_setattr(struct dentry *dentry, struct nfs_fattr *fattr,
 		}
 	}
 
-	/* Deal with open(O_TRUNC) */
-	if (sattr->ia_valid & ATTR_OPEN)
-		sattr->ia_valid &= ~(ATTR_MTIME|ATTR_CTIME|ATTR_OPEN);
-
 	status = nfs4_do_setattr(inode, cred, fattr, sattr, state);
 	if (status == 0)
 		nfs_setattr_update_inode(inode, sattr);