nfp: bpf: allow source ptr type be map ptr in memcpy optimization

Map read has been supported on NFP, this patch enables optimization
for memcpy from map to packet.

This patch also fixed one latent bug which will cause copying from
unexpected address once memcpy for map pointer enabled.  The fixed
code path was not exercised before.

Reported-by: Mary Pham <mary.pham@netronome.com>
Reported-by: David Beckett <david.beckett@netronome.com>
Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>