mod.h: make consistent across all three platforms

mod.h only exists to include the platform-specific mod_os.h, so we can
get rid of it and just call the platform header mod.h.

Then, create a libspl mod.h, and move the relevant items to it so we can
start building on it.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17537

lib/libspl/include/Makefile.am

@@ -45,6 +45,7 @@ libspl_sys_HEADERS = \
 	%D%/sys/list_impl.h \
 	%D%/sys/mhd.h \
 	%D%/sys/mkdev.h \
+	%D%/sys/mod.h \
 	%D%/sys/policy.h \
 	%D%/sys/poll.h \
 	%D%/sys/priv.h \

lib/libspl/include/sys/mod.h

@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License (the "License").
+ * You may not use this file except in compliance with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or https://opensource.org/licenses/CDDL-1.0.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+/*
+ * Copyright (c) 2005, 2010, Oracle and/or its affiliates. All rights reserved.
+ * Copyright 2011 Nexenta Systems, Inc.  All rights reserved.
+ * Copyright (c) 2012, 2018 by Delphix. All rights reserved.
+ * Copyright (c) 2012, Joyent, Inc. All rights reserved.
+ */
+
+#ifndef _SYS_MOD_H
+#define	_SYS_MOD_H
+
+#define	ZFS_MODULE_PARAM(scope, prefix, name, type, perm, desc)
+#define	ZFS_MODULE_PARAM_ARGS void
+#define	ZFS_MODULE_PARAM_CALL(scope_prefix, name_prefix, name, setfunc, \
+	getfunc, perm, desc)
+
+#define	EXPORT_SYMBOL(x)
+
+#endif