Btrfs: determine level of old roots

In btrfs_find_all_roots' termination condition, we compare the level of the
old buffer we got from btrfs_search_old_slot to the level of the current
root node. We'd better compare it to the level of the rewinded root node.

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

diff --cc fs/btrfs/backref.c
Simple merge

diff --cc fs/btrfs/ctree.c
Simple merge

diff --cc fs/btrfs/ctree.h
Simple merge