Add kern_feature.h header file

This file evens up kernel differences depending on versions and
distributions.

This initial version of kern_feature.h includes declaration of
truncate_inode_pages_final() function which is added to kernel 3.15
and backported to RHEL 7 GA kernel.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

fs/nilfs2/inode.c

@@ -26,6 +26,7 @@
 #include <linux/mpage.h>
 #include <linux/writeback.h>
 #include <linux/aio.h>
+#include "kern_feature.h"
 #include "nilfs.h"
 #include "btnode.h"
 #include "segment.h"

fs/nilfs2/kern_feature.h

@@ -0,0 +1,56 @@
+/*
+ * kern_feature.h - Kernel dependent features
+ *
+ * Licensed under GPLv2: the complete text of the GNU General Public
+ * License can be found in COPYING file of the nilfs-kmod package.
+ */
+
+#ifndef NILFS_KERN_FEATURE_H
+#define NILFS_KERN_FEATURE_H
+
+#include <linux/version.h>
+
+/*
+ * Please define as 0/1 here if you want to override
+ */
+
+/*
+ * for Red Hat Enterprise Linux 7.x (and clones like CentOS or SL)
+ */
+#if defined(RHEL_MAJOR) && (RHEL_MAJOR == 7)
+#  define	HAVE_TRUNCATE_INODE_PAGES_FINAL	1
+#endif
+
+/*
+ * defaults
+ */
+
+/*
+ * defaults dependent to kernel versions
+ */
+#ifdef LINUX_VERSION_CODE
+/*
+ * linux-3.15 and later kernels have truncate_inode_pages_final()
+ * and use it in ->evict_inode() implementation.
+ */
+#ifndef HAVE_TRUNCATE_INODE_PAGES_FINAL
+# define HAVE_TRUNCATE_INODE_PAGES_FINAL \
+	(LINUX_VERSION_CODE >= KERNEL_VERSION(3, 15, 0))
+#endif
+#endif /* LINUX_VERSION_CODE */
+
+
+#include <linux/fs.h>
+
+/*
+ * definitions dependent to above macros
+ */
+#if !HAVE_TRUNCATE_INODE_PAGES_FINAL
+static inline void truncate_inode_pages_final(struct address_space *mapping)
+{
+	if (mapping->nrpages)
+		truncate_inode_pages(mapping, 0);
+}
+#endif
+
+#endif /* NILFS_KERN_FEATURE_H */