kernel: audit: beautify code, for extern function, better to check its parameters by itself

  __audit_socketcall is an extern function.
  better to check its parameters by itself.

    also can return error code, when fail (find invalid parameters).
    also use macro instead of real hard code number
    also give related comments for it.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
[eparis: fix the return value when !CONFIG_AUDIT]
Signed-off-by: Eric Paris <eparis@redhat.com>
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index b59ffb2..d57ad32 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -226,7 +226,7 @@
 	union {
 		struct {
 			int nargs;
-			long args[6];
+			long args[AUDITSC_ARGS];
 		} socketcall;
 		struct {
 			kuid_t			uid;
@@ -2491,17 +2491,20 @@
 
 /**
  * audit_socketcall - record audit data for sys_socketcall
- * @nargs: number of args
+ * @nargs: number of args, which should not be more than AUDITSC_ARGS.
  * @args: args array
  *
  */
-void __audit_socketcall(int nargs, unsigned long *args)
+int __audit_socketcall(int nargs, unsigned long *args)
 {
 	struct audit_context *context = current->audit_context;
 
+	if (nargs <= 0 || nargs > AUDITSC_ARGS || !args)
+		return -EINVAL;
 	context->type = AUDIT_SOCKETCALL;
 	context->socketcall.nargs = nargs;
 	memcpy(context->socketcall.args, args, nargs * sizeof(unsigned long));
+	return 0;
 }
 
 /**