netfilter: nft_tunnel: add the missing nla_nest_cancel()

When nla_put_xxx() fails under nla_nest_start_noflag(),
nla_nest_cancel() should be called, so that the skb can
be trimmed properly.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>