evaluate: reject set in concatenation

evaluate: reject set in concatenation

Consider the following ruleset.

 define ext_if = { "eth0", "eth1" }
 table ip filter {
    chain c {
        iifname . tcp dport { $ext_if . 22 } accept
    }
 }

Attempting to load this ruleset results in:

BUG: invalid expression type 'set' in setnft: netlink.c:304: __netlink_gen_concat_key: Assertion `0' failed.
Aborted (core dumped)

After this patch:

 # nft -f ruleset.nft
 ruleset.nft:1:17-40: Error: cannot use set in concatenation
 define ext_if = { "eth0", "eth1" }
                 ^^^^^^^^^^^^^^^^^^

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1715
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>