Fix free memory calculation on v3.14+

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

2016-07-28 599d0c95 mm, vmscan: move LRU lists to node

That 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.

This was /nearly/ fixed in ZoL by:

2017-08-24 2209e40 Linux 4.8+ compatibility fix for vm stats

...which put in place the config infrastructure to detect the API change
and fix an instance of NR_FILE_PAGES in arc_evictable_memory().
Unfortunately the free memory calculation at that time was done in SPL
freemem, and the API fix wasn't applied to the SPL. As a result our free
memory calculation remained broken until it was again /nearly/ fixed by:

2017-09-16 787acae Linux 3.14 compat: IO acct, global_page_state, etc
2017-09-19 661907e Linux 4.14 compat: IO acct, global_page_state, etc

These two patches copy SPL freemem to arc_free_memory() and do the
global_node_page_state() split, /almost/ correctly...

Unfortunately they assume NR_SLAB_RECLAIMABLE moved to
global_node_page_state() along with NR_INACTIVE_FILE and
NR_INACTIVE_ANON, but NR_SLAB_RECLAIMABLE actually stayed in
global_page_state(), thus leaving our free memory calculation broken for
19 months on Linux v3.14+ - until now!

Additionally, Linux 4.14 changes global_page_state() to
global_zone_page_state(), so fix that as well

Signed-off-by: Chris Dunlop <chris@onthe.net.au>

config/kernel-global_zone_page_state.m4

@@ -0,0 +1,21 @@
+dnl #
+dnl # 4.14 API change
+dnl #
+dnl # linux c41f012ade
+dnl # mm: rename global_page_state to global_zone_page_state
+dnl #
+AC_DEFUN([ZFS_AC_KERNEL_GLOBAL_ZONE_PAGE_STATE], [
+	AC_MSG_CHECKING([whether to use global_zone_page_state])
+	ZFS_LINUX_TRY_COMPILE([
+		#include <linux/mm.h>
+		#include <linux/vmstat.h>
+        ],[
+			(void) global_zone_page_state(0);
+	],[
+		AC_MSG_RESULT(yes)
+		AC_DEFINE(ZFS_GLOBAL_ZONE_PAGE_STATE, 1,
+			[using global_zone_page_state()])
+	],[
+		AC_MSG_RESULT(no)
+	])
+])

config/kernel.m4

@@ -124,6 +124,7 @@ AC_DEFUN([ZFS_AC_CONFIG_KERNEL], [
 	ZFS_AC_KERNEL_HAVE_GENERIC_SETXATTR
 	ZFS_AC_KERNEL_CURRENT_TIME
 	ZFS_AC_KERNEL_VM_NODE_STAT
+	ZFS_AC_KERNEL_GLOBAL_ZONE_PAGE_STATE
 
 	AS_IF([test "$LINUX_OBJ" != "$LINUX"], [
 		KERNEL_MAKE="$KERNEL_MAKE O=$LINUX_OBJ"

module/zfs/arc.c

@@ -4701,9 +4701,15 @@ arc_free_memory(void)
 #else
 #ifdef ZFS_GLOBAL_NODE_PAGE_STATE
 	return (ptob(nr_free_pages() +
+	    /* node stats */
 	    global_node_page_state(NR_INACTIVE_FILE) +
 	    global_node_page_state(NR_INACTIVE_ANON) +
-	    global_node_page_state(NR_SLAB_RECLAIMABLE)));
+	    /* zone stats */
+#ifdef ZFS_GLOBAL_ZONE_PAGE_STATE
+	    global_zone_page_state(NR_SLAB_RECLAIMABLE)));
+#else
+	    global_page_state(NR_SLAB_RECLAIMABLE)));
+#endif /* ZFS_GLOBAL_ZONE_PAGE_STATE */
 #else
 	return (ptob(nr_free_pages() +
 	    global_page_state(NR_INACTIVE_FILE) +