btrfs: don't use the extent_root in flush_space

We only need the root to start a transaction, and since it's a global
root we can pick anything, change to the tree_root as we'll have a lot
of extent roots in the future.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>