evaluate: avoid reference to multiple src data in statements which set values

evaluate: avoid reference to multiple src data in statements which set values

Prevent this assert:

% nft [..] tcp dport set { 0 , 1 }
BUG: unknown expression type set reference
nft: netlink_linearize.c:696: netlink_gen_expr: Assertion `0' failed.
Aborted

We can't use a set here because we will not known which value to use.

With this patch, a proper error message is reported to users:

% nft add rule t c tcp dport set {1, 2, 3, 4, 5}
<cmdline>:1:28-42: Error: you cannot use a set here, unknown value to use
add rule t c tcp dport set {1, 2, 3, 4, 5}
             ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^

% nft add rule t c tcp dport set @s
<cmdline>:1:28-29: Error: you cannot reference a set here, unknown value to use
add rule t c tcp dport set @s
             ~~~~~~~~~~~~~~^^

This error is reported to all statements which set values.

Signed-off-by: Arturo Borrero Gonzalez <arturo@debian.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>