Btrfs: tree mod log's old roots could still be part of the tree

Btrfs: tree mod log's old roots could still be part of the tree

Tree mod log treated old root buffers as always empty buffers when starting
the rewind operations. However, the old root may still be part of the
current tree at a lower level, with still some valid entries.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>