ABD optimized page allocation code

* Convert ABD to use the Linux Kernel scatterlist implementation
  instead of the hand rolled one from illumos.

* Scatter ABDs are preferentially populated with higher order
  compound pages from a single zone.  Allocation size is
  progressively decreased until it can be satisfied without
  performing reclaim or compaction.

* An alternate page allocator is provided for kernels older
  than 3.6 and for CONFIG_HIGHMEM systems.  This allocator
  is designed as a fallback for maximum compatibility.

* Extended abdstats to provide visibility in the the allocator.

* Add cached value for PAGESIZE in userspace.

Contributions-by:
Chunwei Chen <david.chen@osnexus.com>
Gvozden Neskovic <neskovic@gmail.com>
Jinshan Xiong <jinshan.xiong@intel.com>
Isaac Huang <he.huang@intel.com>
David Quigley <david.quigley@intel.com>
Brian Behlendorf <behlendorf1@llnl.gov>

lib/libspl/Makefile.am

@@ -24,6 +24,7 @@ USER_C = \
 	getmntany.c \
 	list.c \
 	mkdirp.c \
+	page.c \
 	strlcat.c \
 	strlcpy.c \
 	strnlen.c \

lib/libspl/include/sys/param.h

@@ -57,8 +57,11 @@
 #define	MAXUID		UINT32_MAX	/* max user id */
 #define	MAXPROJID	MAXUID		/* max project id */
 
-#ifndef	PAGESIZE
-#define	PAGESIZE	(sysconf(_SC_PAGESIZE))
+#ifdef	PAGESIZE
+#undef	PAGESIZE
 #endif /* PAGESIZE */
 
+extern size_t spl_pagesize(void);
+#define	PAGESIZE	(spl_pagesize())
+
 #endif

lib/libspl/page.c

@@ -0,0 +1,34 @@
+/*
+ * CDDL HEADER START
+ *
+ * The contents of this file are subject to the terms of the
+ * Common Development and Distribution License, Version 1.0 only
+ * (the "License").  You may not use this file except in compliance
+ * with the License.
+ *
+ * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
+ * or http://www.opensolaris.org/os/licensing.
+ * See the License for the specific language governing permissions
+ * and limitations under the License.
+ *
+ * When distributing Covered Code, include this CDDL HEADER in each
+ * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
+ * If applicable, add the following below this CDDL HEADER, with the
+ * fields enclosed by brackets "[]" replaced with your own identifying
+ * information: Portions Copyright [yyyy] [name of copyright owner]
+ *
+ * CDDL HEADER END
+ */
+
+#include <unistd.h>
+
+size_t pagesize = 0;
+
+size_t
+spl_pagesize(void)
+{
+	if (pagesize == 0)
+		pagesize = sysconf(_SC_PAGESIZE);
+
+	return (pagesize);
+}