net: ipv6: unexport __init-annotated seg6_hmac_net_init()

As of commit 5801f064 ("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.

This remove the EXPORT_SYMBOL to fix modpost warning:

WARNING: modpost: vmlinux.o(___ksymtab+seg6_hmac_net_init+0x0): Section mismatch in reference from the variable __ksymtab_seg6_hmac_net_init to the function .init.text:seg6_hmac_net_init()
The symbol seg6_hmac_net_init is exported and annotated __init
Fix this by removing the __init annotation of seg6_hmac_net_init or drop the export.

Fixes: bf355b8d

 ("ipv6: sr: add core files for SR HMAC support")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20220628033134.21088-1-yuehaibing@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>