nfp: bpf: relax prog rejection for mtu check through max_pkt_offset

MTU change is refused whenever the value of new MTU is bigger than
the max packet bytes that fits in NFP Cluster Target Memory (CTM).
However, an eBPF program doesn't always need to access the whole
packet data.

The maximum direct packet access (DPA) offset has always been
caculated by verifier and stored in the max_pkt_offset field of prog
aux data.

Signed-off-by: Yu Xiao <yu.xiao@corigine.com>
Reviewed-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Reviewed-by: Niklas Soderlund <niklas.soderlund@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>