f2fs: add roll-forward recovery process for encrypted dentry

Add roll-forward recovery process for encrypted dentry, so the first fsync
issued to an encrypted file does not need writing checkpoint.

This improves the performance of the following test at thousands of small
files: open -> write -> fsync -> close

Signed-off-by: Shuoran Liu <liushuoran@huawei.com>
Acked-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: modify kernel message to show encrypted names]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

 Conflicts:
	fs/f2fs/dir.c
	fs/f2fs/f2fs.h