net: ipv6: unexport __init-annotated seg6_hmac_init()

In the Linux kernel, the following vulnerability has been resolved:

net: ipv6: unexport __init-annotated seg6_hmac_init()

EXPORT_SYMBOL and __init is a bad combination because the .init.text

section is freed up after the initialization. Hence, modules cannot

use symbols annotated __init. The access to a freed symbol may end up

with kernel panic.

modpost used to detect it, but it has been broken for a decade.

Recently, I fixed modpost so it started to warn it again, then this

showed up in linux-next builds.

There are two ways to fix it:

- Remove __init

- Remove EXPORT_SYMBOL

I chose the latter for this case because the caller (net/ipv6/seg6.c)

and the callee (net/ipv6/seg6_hmac.c) belong to the same module.

It seems an internal function call in ipv6.ko.