mlxsw: spectrum_acl: Remember where to continue rehash migration

Store pointer to vchunk where the migration was interrupted, as well as
ventry pointer to start from and to stop at (during rollback). This
saved pointers need to be forgotten in case of ventries list or vchunk
list changes, which is done by couple of "changed" helpers.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>