rtnl_neightbl: always decode struct ndt_config and struct ndt_stats

rtnl_neightbl: always decode struct ndt_config and struct ndt_stats

* rtnl_neightbl.c (struct_ndt_config, struct_ndt_stats): New typedefs.
[HAVE_STRUCT_NDT_CONFIG]: New static_assert to check
that sizeof(struct ndt_config) has the expected value.
[HAVE_STRUCT_NDT_STATS]: New static_assert to check
that sizeof(struct ndt_stats) has the expected value.
(decode_ndt_config) [HAVE_STRUCT_NDT_CONFIG]: Remove guard.
(decode_ndt_config): Change the type of ndtc variable
to struct_ndt_config.
(decode_ndt_stats) [HAVE_STRUCT_NDT_STATS]: Remove guard.
(decode_ndt_stats): Change the type of ndtst variable
to struct_ndt_stats.

References: https://bugzilla.redhat.com/show_bug.cgi?id=1758201