f2fs: use extent_cache by default

We don't need to handle the duplicate extent information.

The integrated rule is:
 - update on-disk extent with largest one tracked by in-memory extent_cache
 - destroy extent_tree for the truncation case
 - drop per-inode extent_cache by shrinker

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

fs/f2fs/shrinker.c

@@ -117,6 +117,8 @@ void f2fs_join_shrinker(struct f2fs_sb_info *sbi)
 
 void f2fs_leave_shrinker(struct f2fs_sb_info *sbi)
 {
+	f2fs_shrink_extent_tree(sbi, __count_extent_cache(sbi));
+
 	spin_lock(&f2fs_list_lock);
 	list_del(&sbi->s_list);
 	spin_unlock(&f2fs_list_lock);