Fix free memory calculation on v3.14+

Provide infrastructure to auto-configure to enum and API changes in the
global page stats used for our free memory calculations.

arc_free_memory has been broken since an API change in Linux v3.14:

2016-07-28 v4.8 599d0c95 mm, vmscan: move LRU lists to node
2016-07-28 v4.8 75ef7184 mm, vmstat: add infrastructure for per-node
  vmstats

These commits moved some of global_page_state() into
global_node_page_state(). The API change was particularly egregious as,
instead of breaking the old code, it silently did the wrong thing and we
continued using global_page_state() where we should have been using
global_node_page_state(), thus indexing into the wrong array via
NR_SLAB_RECLAIMABLE et al.

There have been further API changes along the way:

2017-07-06 v4.13 385386cf mm: vmstat: move slab statistics from zone to
  node counters
2017-09-06 v4.14 c41f012a mm: rename global_page_state to
  global_zone_page_state

...and various (incomplete, as it turns out) attempts to accomodate
these changes in ZoL:

2017-08-24 2209e409 Linux 4.8+ compatibility fix for vm stats
2017-09-16 787acae0 Linux 3.14 compat: IO acct, global_page_state, etc
2017-09-19 661907e6 Linux 4.14 compat: IO acct, global_page_state, etc

The config infrastructure provided here resolves these issues going back
to the original API change in v3.14 and is robust against further Linux
changes in this area.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Closes #7170

scripts/Makefile.am

@@ -5,6 +5,7 @@ EXTRA_DIST = dkms.mkconf dkms.postbuild kmodtool zfs2zol-patch.sed cstyle.pl
 pkgdatadir = $(datadir)/@PACKAGE@
 dist_pkgdata_SCRIPTS = \
 	$(top_builddir)/scripts/common.sh \
+	$(top_srcdir)/scripts/enum-extract.pl \
 	$(top_srcdir)/scripts/zimport.sh \
 	$(top_srcdir)/scripts/zfs.sh \
 	$(top_srcdir)/scripts/zfs-tests.sh \
@@ -15,3 +16,4 @@ dist_pkgdata_SCRIPTS = \
 	$(top_srcdir)/scripts/zpios-survey.sh \
 	$(top_srcdir)/scripts/smb.sh \
 	$(top_srcdir)/scripts/zfs-helpers.sh
+

scripts/enum-extract.pl

@@ -0,0 +1,58 @@
+#!/usr/bin/perl -w
+
+my $usage = <<EOT;
+usage: config-enum enum [file ...]
+
+Returns the elements from an enum declaration.
+
+"Best effort": we're not building an entire C interpreter here!
+EOT
+
+use warnings;
+use strict;
+use Getopt::Std;
+
+my %opts;
+
+if (!getopts("", \%opts) || @ARGV < 1) {
+	print $usage;
+	exit 2;
+}
+
+my $enum = shift;
+
+my $in_enum = 0;
+
+while (<>) {
+	# comments
+	s/\/\*.*\*\///;
+	if (m/\/\*/) {
+		while ($_ .= <>) {
+			last if s/\/\*.*\*\///s;
+		}
+	}
+
+	# preprocessor stuff
+	next if /^#/;
+
+	# find our enum
+	$in_enum = 1 if s/^\s*enum\s+${enum}(?:\s|$)//;
+	next unless $in_enum;
+
+	# remove explicit values
+	s/\s*=[^,]+,/,/g;
+
+	# extract each identifier
+	while (m/\b([a-z_][a-z0-9_]*)\b/ig) {
+		print $1, "\n";
+	}
+
+	#
+	# don't exit: there may be multiple versions of the same enum, e.g.
+	# inside different #ifdef blocks. Let's explicitly return all of
+	# them and let external tooling deal with it.
+	#
+	$in_enum = 0 if m/}\s*;/;
+}
+
+exit 0;