f2fs: give an option to enable in-place-updates during fsync to users

If user wrote F2FS_IPU_FSYNC:4 in /sys/fs/f2fs/ipu_policy, f2fs_sync_file
only starts to try in-place-updates.
And, if the number of dirty pages is over /sys/fs/f2fs/min_fsync_blocks, it
keeps out-of-order manner. Otherwise, it triggers in-place-updates.

This may be used by storage showing very high random write performance.

For example, it can be used when,

Seq. writes (Data) + wait + Seq. writes (Node)

is pretty much slower than,

Rand. writes (Data)

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 77426c7..af06e22 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -154,12 +154,11 @@
 	trace_f2fs_sync_file_enter(inode);
 
 	/* if fdatasync is triggered, let's do in-place-update */
-	if (datasync)
+	if (get_dirty_pages(inode) <= SM_I(sbi)->min_fsync_blocks)
 		set_inode_flag(fi, FI_NEED_IPU);
-
 	ret = filemap_write_and_wait_range(inode->i_mapping, start, end);
-	if (datasync)
-		clear_inode_flag(fi, FI_NEED_IPU);
+	clear_inode_flag(fi, FI_NEED_IPU);
+
 	if (ret) {
 		trace_f2fs_sync_file_exit(inode, need_cp, datasync, ret);
 		return ret;