net/mlx5e: Avoid using the ipv6 stub in the TC offload neigh update path

Currently we use the global ipv6_stub var to access the ipv6 global
nd table. This practice gets us to troubles when the stub is only partially
set e.g when ipv6 is loaded under the disabled policy. In this case, as of commit
343d60aa ("ipv6: change ipv6_stub_impl.ipv6_dst_lookup to take net argument")
the stub is not null, but stub->nd_tbl is and we crash.

As we can access the ipv6 nd_tbl directly, the fix is just to avoid the
reference through the stub. There is one place in the code where we
issue ipv6 route lookup and keep doing it through the stub, but that
mentioned commit makes sure we get -EAFNOSUPPORT from the stack.

Fixes: 232c0013

 ("net/mlx5e: Add support to neighbour update flow")
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reviewed-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>