netfilter: nf_flow_table: cache mtu in struct flow_offload_tuple

Reduces the number of cache lines touched in the offload forwarding
path. This is safe because PMTU limits are bypassed for the forwarding
path (see commit f87c10a8 for more details).

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>