f2fs: fix incorrect searching position when shrinking extent cache

When shrinking extent cache, we have two steps in the flow:
1) shrink objects which are unreferenced by inodes;
2) shrink objects from LRU list of extent cache.

In step 1, if we haven't shrunk enough number of objects, we will try
step 2, but before that we didn't update the searching position which
may point to last inode index in global extent tree, result in failing
to shrink objects by traversing the all inodes' extent tree.

In this patch, we reset searching position to beginning of global extent
tree for fixing.
Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/extent_cache.c

@@ -652,6 +652,11 @@ unsigned int f2fs_shrink_extent_tree(struct f2fs_sb_info *sbi, int nr_shrink)
 	}
 	spin_unlock(&sbi->extent_lock);
 
+	/*
+	 * reset ino for searching victims from beginning of global extent tree.
+	 */
+	ino = F2FS_ROOT_INO(sbi);
+
 	while ((found = radix_tree_gang_lookup(root,
 				(void **)treevec, ino, EXT_TREE_VEC_SIZE))) {
 		unsigned i;