Make module tunables cross platform

Adds ZFS_MODULE_PARAM to abstract module parameter
setting to operating systems other than Linux.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9230

include/sys/zfs_context.h

@@ -65,6 +65,7 @@
 #include <sys/procfs_list.h>
 #include <linux/dcache_compat.h>
 #include <linux/utsname_compat.h>
+#include <linux/mod_compat.h>
 
 #else /* _KERNEL */
 
@@ -200,6 +201,16 @@ extern int aok;
 	ZFS_PROBE4(#a, (unsigned long)c, (unsigned long)e, (unsigned long)g, \
 	(unsigned long)i)
 
+/*
+ * Tunables.
+ */
+#define	ZFS_MODULE_PARAM(scope_prefix, name_prefix, name, type, perm, desc)
+
+/*
+ * Exported symbols
+ */
+#define	EXPORT_SYMBOL(x)
+
 /*
  * Threads.
  */