f2fs: don't drop dentry pages after fs shutdown

As description in commit "f2fs: don't drop any page on f2fs_cp_error()
case":

"We still provide readdir() after shtudown, so we should keep pages to
avoid additional IOs."

In order to provider lastest directory structure, let's keep dentry
pages in cache after fs shutdown.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>