evaluate: reset eval context when evaluating set definitions

evaluate: reset eval context when evaluating set definitions

David reported nft chokes on this:
nft -f /tmp/A
/tmp/A:9:22-45: Error: datatype mismatch, expected concatenation of (IPv4 address, internet network service, IPv4 address), expression has type concatenation of (IPv4 address, internet network service)
cat /tmp/A
flush ruleset;
table ip filter {
	set setA {
		type ipv4_addr . inet_service . ipv4_addr
		flags timeout
	}
	set setB {
		type ipv4_addr . inet_service
		flags timeout
	}
}

Problem is we leak set definition details of setA to setB via eval
context, so reset this.

Also add test case for this.

Reported-by: David Fabian <david.fabian@bosson.cz>
Signed-off-by: Florian Westphal <fw@strlen.de>