Linux 6.16: remove writepage and readahead_page

Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes #17443

include/os/linux/Makefile.am

@@ -8,6 +8,7 @@ kernel_linux_HEADERS = \
 	%D%/kernel/linux/mm_compat.h \
 	%D%/kernel/linux/mod_compat.h \
 	%D%/kernel/linux/page_compat.h \
+	%D%/kernel/linux/pagemap_compat.h \
 	%D%/kernel/linux/simd.h \
 	%D%/kernel/linux/simd_aarch64.h \
 	%D%/kernel/linux/simd_arm.h \

include/os/linux/kernel/linux/pagemap_compat.h

@@ -0,0 +1,36 @@
+// SPDX-License-Identifier: CDDL-1.0
+/*
+ * 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 https://opensource.org/licenses/CDDL-1.0.
+ * 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) 2025, Rob Norris <robn@despairlabs.com>
+ */
+
+#ifndef _ZFS_PAGEMAP_COMPAT_H
+#define	_ZFS_PAGEMAP_COMPAT_H
+
+#include <linux/pagemap.h>
+
+#ifndef HAVE_PAGEMAP_READAHEAD_PAGE
+#define	readahead_page(ractl) (&(__readahead_folio(ractl)->page))
+#endif
+
+#endif