net: mark some functions in net_util.h as public
net: mark some functions in net_util.h as public

They are not really private to net, since they are used "extensively"
outside of it, so keeping using NET_EXPORT_PRIVATE does not make much
sense and just gives the wrong impression for the occasional reader when
looking at this header file.

BUG=552248
R=eroman@chromium.org

Review URL: https://codereview.chromium.org/1541933003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366492}

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