netfilter: nft_inner: fix return value check in nft_inner_parse_l2l3()

In nft_inner_parse_l2l3(), the return value of skb_header_pointer() is
'veth' instead of 'eth' when case 'htons(ETH_P_8021Q)' and fix it.

Fixes: 3a07327d

 ("netfilter: nft_inner: support for inner tunnel header matching")
Signed-off-by: Peng Wu <wupeng58@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>