MINOR: sample: provide the original sample_conv descriptor struct to the argument checker function.

Note that this argument checker is still unused but will be used by
maps.
diff --git a/src/acl.c b/src/acl.c
index 9f487a3..9ea082a 100644
--- a/src/acl.c
+++ b/src/acl.c
@@ -1277,7 +1277,7 @@
 					if (!conv_expr->arg_p)
 						conv_expr->arg_p = empty_arg_list;
 
-					if (conv->val_args && !conv->val_args(conv_expr->arg_p, &err_msg)) {
+					if (conv->val_args && !conv->val_args(conv_expr->arg_p, conv, &err_msg)) {
 						memprintf(err, "ACL keyword '%s' : invalid args in conv method '%s' : %s.",
 						          expr->kw, ckw, err_msg);
 						free(err_msg);