Add tunables for channel programs

This patch adds tunables for modifying the maximum memory limit and
maximum instruction limit that can be specified when running a channel
program.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov
Reviewed-by: Sara Hartse <sara.hartse@delphix.com>
Signed-off-by: John Gallagher <john.gallagher@delphix.com>
External-issue: LX-1085
Closes #7618

man/man5/zfs-module-parameters.5

@@ -1,6 +1,7 @@
 '\" te
 .\" Copyright (c) 2013 by Turbo Fredriksson <turbo@bayour.com>. All rights reserved.
 .\" Copyright (c) 2017 Datto Inc.
+.\" Copyright (c) 2018 by Delphix. All rights reserved.
 .\" 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
@@ -1560,6 +1561,30 @@ dataset being written to had the property setting \fBlogbias=throughput\fR.
 Default value: \fB32,768\fR.
 .RE
 
+.sp
+.ne 2
+.na
+\fBzfs_lua_max_instrlimit\fR (ulong)
+.ad
+.RS 12n
+The maximum execution time limit that can be set for a ZFS channel program,
+specified as a number of Lua instructions.
+.sp
+Default value: \fB100,000,000\fR.
+.RE
+
+.sp
+.ne 2
+.na
+\fBzfs_lua_max_memlimit\fR (ulong)
+.ad
+.RS 12n
+The maximum memory limit that can be set for a ZFS channel program, specified
+in bytes.
+.sp
+Default value: \fB104,857,600\fR.
+.RE
+
 .sp
 .ne 2
 .na