nfp: bpf: factor out is_mbpf_load & is_mbpf_store

It is usual that we need to check if one BPF insn is for loading/storeing
data from/to memory.

Therefore, it makes sense to factor out related code to become common
helper functions.

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