f2fs: fix to update age extent correctly during truncation

nr_free may be less than len, we should update age extent cache
w/ range [fofs, len] rather than [fofs, nr_free].

Fixes: 71644dff ("f2fs: add block_age-based extent cache")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>