f2fs: use generic terms used for encrypted block management

commit d4c759ee5faa51e0b0ee55d8a229ba5b80c4917e upstream.

This patch renames functions regarding to buffer management via META_MAPPING
used for encrypted blocks especially. We can actually use them in generic way.

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