Btrfs: Fix balance_level to free the middle block if there is room in the left one

balance level starts by trying to empty the middle block, and then
pushes from the right to the middle.  This might empty the right block
and leave a small number of pointers in the middle.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index cc0d7f3..c49592c 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -306,13 +306,13 @@
 	int bit;
 	int ret;
 	int full_search = 0;
-	int factor = 8;
+	int factor = 10;
 
 	block_group_cache = &info->block_group_cache;
 	total_fs_bytes = btrfs_super_total_bytes(&root->fs_info->super_copy);
 
 	if (!owner)
-		factor = 8;
+		factor = 10;
 
 	bit = block_group_state_bits(data);