Merge branch 'use network helpers, part 4'

Geliang Tang says:

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From: Geliang Tang <tanggeliang@kylinos.cn>

This patchset adds post_socket_cb pointer into
struct network_helper_opts to make start_server_addr() helper
more flexible. With these modifications, many duplicate codes
can be dropped.

Patches 1-3 address Martin's comments in the previous series.
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Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>