net: Refactor xdp_convert_buff_to_frame

Move the guts of xdp_convert_buff_to_frame to a new helper,
xdp_update_frame_from_buff so it can be reused removing code duplication

Suggested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Co-developed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: David Ahern <dsahern@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/90a68c283d7ebeb48924934c9b7ac79492300472.1594734381.git.lorenzo@kernel.org