UPSTREAM: f2fs: fix wrong calculation of block age

Currently we wrongly calculate the new block age to
old * LAST_AGE_WEIGHT / 100.

Fix it to new * (100 - LAST_AGE_WEIGHT) / 100
                + old * LAST_AGE_WEIGHT / 100.

Signed-off-by: qixiaoyu1 <qixiaoyu1@xiaomi.com>
Signed-off-by: xiongping1 <xiongping1@xiaomi.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
(cherry picked from commit b03a41a495df35f8e8d25220878bd6b8472d9396)
Change-Id: I989877caf2517245750e7ed809b2af71f8943f96
Signed-off-by: zhaoyuenan <amktiao030215@gmail.com>