btrfs: clear BTRFS_ROOT_DEAD_RELOC_TREE before dropping the reloc root

We were doing the clear dance for the reloc root after doing the drop of
the reloc root, which means we have a giant window where we could miss
having BTRFS_ROOT_DEAD_RELOC_TREE unset and the reloc_root == NULL.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>