Allow the VM to provide backpressure on the ARC cache as it does

on Solaris.

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c

@@ -131,6 +131,8 @@
 #include <sys/kstat.h>
 #include <sys/sdt.h>
 
+#include <vm/vm_pageout.h>
+
 static kmutex_t		arc_reclaim_thr_lock;
 static kcondvar_t	arc_reclaim_thr_cv;	/* used to signal reclaim thr */
 static uint8_t		arc_thread_exit;
@@ -1809,6 +1811,13 @@ arc_reclaim_needed(void)
 
 #ifdef _KERNEL
 
+	/*
+	 * If pages are needed or we're within 2048 pages 
+	 * of needing to page need to reclaim
+	 */
+	if (vm_pages_needed || (vm_paging_target() > -2048))
+		return (1);
+
 	if (needfree)
 		return (1);