evaluate: string prefix expression must retain original length

evaluate: string prefix expression must retain original length

To make something like "eth*" work for interval sets (match
eth0, eth1, and so on...) we must treat the string as a 128 bit
integer.

Without this, segtree will do the wrong thing when applying the prefix,
because we generate the prefix based on 'eth*' as input, with a length of 3.

The correct import needs to be done on "eth\0\0\0\0\0\0\0...", i.e., if
the input buffer were an ipv6 address, it should look like "eth\0::",
not "::eth".

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>