btrfs: add assertions for releasing trans handle reservations

These are useful for debugging problems where we mess with
trans->block_rsv to make sure we're not screwing something up.

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