box: pass statement being rolled back (if any) to `priv_grant`

In scope of #9893 we are going to run statement `on_rollback` triggers
before rolling back the corresponding statement. During rollback of DDL in
the `_priv` space, the database is accessed from `user_reload_privs` to
reload user privileges, so we need it to account for the current statement
being rolled back: i.e., the new tuple that was introduced (if any) must
not be used, while the old tuple (if any) must be used.

Needed for #9893

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