netfilter: ipset: fix interface comparision in hash-netiface sets

ifname_compare() assumes that skb->dev is zero-padded,
e.g 'eth1\0\0\0\0\0...'. This isn't always the case. e1000 driver does

strncpy(netdev->name, pci_name(pdev), sizeof(netdev->name) - 1);

in e1000_probe(), so once device is registered dev->name memory contains
'eth1\0:0:3\0\0\0' (or something like that), which makes eth1 compare
fail.

Use plain strcmp() instead.

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