rule.Rule.Build: Don't assume that no syscalls means all syscalls

Rule.Build assumes that if no syscalls are specified they all are set.
This is really only the case when the exit list is used since the
syscall numbers aren't available in the other lists.  When we assume
that all of the syscalls are enabled, we end up generating wireformat
rules for e.g. 'task,never' that have all of the syscall bits set.  That
doesn't match what is already used when 'auditctl -a task,never' is
used.  It may be ignored by the kernel when such a rule is added, but
it would cause problems when that rule is deleted.

rule/rule.go

@@ -41,14 +41,22 @@ const (
 
 // Build builds an audit rule.
 func Build(rule Rule) (WireFormat, error) {
-	data := &ruleData{allSyscalls: true}
+	data := &ruleData{}
 	var err error
 
 	switch v := rule.(type) {
 	case *SyscallRule:
 		if err = data.setList(v.List); err != nil {
 			return nil, err
 		}
+
+		// While it's possible to set syscalls on lists other than the 'exit' list
+		// they don't actually do anything since the syscall information isn't
+		// available at that time.  Don't assume that all syscalls are enabled.
+		if data.flags == exitFilter {
+			data.allSyscalls = true
+		}
+
 		if err = data.setAction(v.Action); err != nil {
 			return nil, err
 		}