Fix aarch64 build

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

include/linux/Makefile.am

@@ -10,7 +10,8 @@ KERNEL_H = \
 	$(top_srcdir)/include/linux/simd_x86.h \
 	$(top_srcdir)/include/linux/simd_aarch64.h \
 	$(top_srcdir)/include/linux/mod_compat.h \
-	$(top_srcdir)/include/linux/page_compat.h
+	$(top_srcdir)/include/linux/page_compat.h \
+	$(top_srcdir)/include/linux/compiler_compat.h
 
 USER_H =
 

include/linux/compiler_compat.h

@@ -0,0 +1,35 @@
+/*
+ * CDDL HEADER START
+ *
+ * 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 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
+ */
+
+/*
+ * Copyright (C) 2018 Lawrence Livermore National Security, LLC.
+ */
+
+#ifndef _ZFS_COMPILER_COMPAT_H
+#define	_ZFS_COMPILER_COMPAT_H
+
+#include <linux/compiler.h>
+
+#if !defined(READ_ONCE)
+#define	READ_ONCE(x)		ACCESS_ONCE(x)
+#endif
+
+#endif	/* _ZFS_COMPILER_COMPAT_H */

include/spl/sys/mutex.h

@@ -28,6 +28,7 @@
 #include <sys/types.h>
 #include <linux/mutex.h>
 #include <linux/lockdep.h>
+#include <linux/compiler_compat.h>
 
 typedef enum {
 	MUTEX_DEFAULT	= 0,

module/spl/spl-condvar.c

@@ -27,6 +27,7 @@
 #include <sys/condvar.h>
 #include <sys/time.h>
 #include <linux/hrtimer.h>
+#include <linux/compiler_compat.h>
 
 void
 __cv_init(kcondvar_t *cvp, char *name, kcv_type_t type, void *arg)