memory: New AS helper to serialize destroy+free

memory: New AS helper to serialize destroy+free

If an AddressSpace has been created in its own allocated
memory, cleaning it up requires first destroying the AS
and then freeing the memory. Doing this doesn't work:

    address_space_destroy(as);
    g_free_rcu(as, rcu);

because both address_space_destroy() and g_free_rcu()
try to use the same 'rcu' node in the AddressSpace struct
and the address_space_destroy hook gets overwritten.

Provide a new address_space_destroy_free() function which
will destroy the AS and then free the memory it uses, all
in one RCU callback.

(CC to stable because the next commit needs this function.)

Cc: qemu-stable@nongnu.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250929144228.1994037-3-peter.maydell@linaro.org
Signed-off-by: Peter Xu <peterx@redhat.com>