f2fs: avoid small data writes by skipping writepages

This patch introduces nr_pages_to_skip(sbi, type) to determine writepages can
be skipped.
The dentry, node, and meta pages can be conrolled by F2FS without breaking the
FS consistency.

Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
index 4c0e98d..1f52b70 100644
--- a/fs/f2fs/checkpoint.c
+++ b/fs/f2fs/checkpoint.c
@@ -187,7 +187,7 @@
 				struct writeback_control *wbc)
 {
 	struct f2fs_sb_info *sbi = F2FS_SB(mapping->host->i_sb);
-	int nrpages = MAX_BIO_BLOCKS(max_hw_blocks(sbi));
+	int nrpages = nr_pages_to_skip(sbi, META);
 	long written;
 
 	if (wbc->for_kupdate)
@@ -682,7 +682,7 @@
 	inode = igrab(entry->inode);
 	spin_unlock(&sbi->dir_inode_lock);
 	if (inode) {
-		filemap_flush(inode->i_mapping);
+		filemap_fdatawrite(inode->i_mapping);
 		iput(inode);
 	} else {
 		/*