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/net/socket.c b/net/socket.c
index 2ca51c7..1bbc37b 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -2436,7 +2436,7 @@
 
 SYSCALL_DEFINE2(socketcall, int, call, unsigned long __user *, args)
 {
-	unsigned long a[6];
+	unsigned long a[AUDITSC_ARGS];
 	unsigned long a0, a1;
 	int err;
 	unsigned int len;
@@ -2452,7 +2452,9 @@
 	if (copy_from_user(a, args, len))
 		return -EFAULT;
 
-	audit_socketcall(nargs[call] / sizeof(unsigned long), a);
+	err = audit_socketcall(nargs[call] / sizeof(unsigned long), a);
+	if (err)
+		return err;
 
 	a0 = a[0];
 	a1 = a[1];