This fixes the panic caused by lock recursion when the grant-table free callbacks are used.
The cause of the recursion is: check_free_callbacks() is always called with the lock gnttab_list_lock held. So, the callback function is called with the lock held. So, when the client uses any of the grant reference allocation methods get_free_entries() is called, which tries to acquire gnttab_list_lock, causing a recursion on the non-recursive lock.