f2fs: merge {invalidate,release}page for meta/node/data pages

This patch merges ->{invalidate,release}page function for meta/node/data pages.

After this, duplication of codes could be removed.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>