evaluate: fix get element for concatenated set

evaluate: fix get element for concatenated set

given:
table ip filter {
	set test {
		type ipv4_addr . ether_addr . mark
		flags interval
		elements = { 198.51.100.0/25 . 00:0b:0c:ca:cc:10-c1:a0:c1:cc:10:00 . 0x0000006f, }
	}
}

We get lookup failure:

nft get element ip filter test { 198.51.100.1 . 00:0b:0c:ca:cc:10 . 0x6f }
Error: Could not process rule: No such file or directory

Its possible to work around this via dummy range somewhere in the key, e.g.
nft get element ip filter test { 198.51.100.1 . 00:0b:0c:ca:cc:10 . 0x6f-0x6f }

but that shouldn't be needed, so make sure the INTERVAL flag is enabled
for the queried element if the set is of interval type.

Signed-off-by: Florian Westphal <fw@strlen.de>