bpf: add comments to BPF ld/ldx sizes

Doc BPF ld/ldx size defines as comments in code, as it makes in
faster to lookup in a programming/review setting, than looking up
the sizes in Documentation/networking/filter.txt.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>