acct: check FMODE_CAN_WRITE

it's not calling ->write() directly anymore.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

kernel/acct.c

@@ -213,7 +213,7 @@ static int acct_on(struct filename *pathname)
 		return -EACCES;
 	}
 
-	if (!file->f_op->write) {
+	if (!(file->f_mode & FMODE_CAN_WRITE)) {
 		kfree(acct);
 		filp_close(file, NULL);
 		return -EIO;