When rip_input() is called as inetsw[].pr_input, rip_iput() is always called
with holding softnet_lock, that is, in case of !defined(NET_MPSAFE) it is
acquired in ipintr(), otherwise(defined(NET_MPSAFE)) it is acquire in
PR_WRAP_INPUT macro.
However, some function calls rip_input() directly without holding softnet_lock.
That causes assertion failure in sbappendaddr().
rip6_input() and icmp6_rip6_input() are also required softnet_lock for the same
reason.
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