f2fs: speed up shrinking extent tree entries

If there is no candidates for shrinking slab entries, we don't need to traverse
any trees at all.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
[Jaegeuk Kim: fix missing initialization reported by Yunlei He]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/shrinker.c b/fs/f2fs/shrinker.c
index a11e099..93606f2 100644
--- a/fs/f2fs/shrinker.c
+++ b/fs/f2fs/shrinker.c
@@ -32,7 +32,7 @@
 
 static unsigned long __count_extent_cache(struct f2fs_sb_info *sbi)
 {
-	return atomic_read(&sbi->total_ext_tree) +
+	return atomic_read(&sbi->total_zombie_tree) +
 				atomic_read(&sbi->total_ext_node);
 }