evaluate: Allow concatenation of rt nexthop etc.

evaluate: Allow concatenation of rt nexthop etc.

Concatenations of rt nexthop or ct {orignal | reply} {saddr | daddr} fail
due to

 # nft add rule ip filter postrouting flow table acct \{ ip saddr . rt nexthop counter \}
 <cmdline>:1:61-70: Error: can not use variable sized data types (invalid) in concat expressions
 add rule ip filter postrouting flow table acct { ip saddr . rt nexthop counter }
                                                  ~~~~~~~~~~~^^^^^^^^^^

Fix this by reordering the check for variable size data types in
expr_evaluate_concat() to happen after expr_evaluate() has been called (via
list_member_evaluate()) for the sub expression. This allows
expr_evaluate_[cr]t() to call [cr]t_expr_update_type() and set the data type
before the check.

Signed-off-by: Anders K. Pedersen <akp@cohaesio.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>