Revert "f2fs: do not skip dentry block writes"

The periodic checkpoint can resolve the previous issue.
So, now we can use this again to improve the reported performance regression:

https://lkml.org/lkml/2015/10/8/20

This reverts commit 15bec0ff5a9ba6d203178fa8772259df6207942a.
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index a903423..bc04e92 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -1340,6 +1340,11 @@
 	if (!get_dirty_pages(inode) && wbc->sync_mode == WB_SYNC_NONE)
 		return 0;
 
+	if (S_ISDIR(inode->i_mode) && wbc->sync_mode == WB_SYNC_NONE &&
+			get_dirty_pages(inode) < nr_pages_to_skip(sbi, DATA) &&
+			available_free_memory(sbi, DIRTY_DENTS))
+		goto skip_write;
+
 	/* during POR, we don't need to trigger writepage at all. */
 	if (unlikely(is_sbi_flag_set(sbi, SBI_POR_DOING)))
 		goto skip_write;