nfp: bpf: don't allow changing MTU above BPF offload limit when active

When BPF offload is active we need may need to restrict the MTU
changes more than just to the limitation of the kernel XDP datapath.
Allow the BPF code to veto a MTU change.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>