Enforce net epoch in in6_selectsrc().

[melifaro]

Summary

in6_selectsrc() may call fib6_lookup() in some cases. The latter function requires epoch.
Some callers of in6_selectsrc() already call it under epoch.
Given the fact that it is preferable not to re-enter epoch multiple times, make the remaining in6_selectsrc() callers call it within net epoch.

Finally, mark it as requiring epoch by adding NET_EPOCH_ASSERT().

Note that in6_selectsrc() is used indirectly by 2 wrappers (socket/non-socket-based access): in6_selectsrc_socket() and inet6_selectsrc_addr().

The following callers currently exists in kernel:

sys/fs/nfsclient/nfs_clport.c: error = in6_selectsrc_addr(fibnum, &sin6->sin6_addr,
^ Wrapped

sys/netpfil/ipfw/nat64/nat64_translate.c: if (in6_selectsrc_addr(M_GETFIB(n), &ip6->ip6_src, 0,
^ Assume datapath is under epoch

sys/netinet6/raw_ip6.c: error = in6_selectsrc_socket(dstsock, optp, inp, so->so_cred,
^ Wrapped
sys/netinet6/raw_ip6.c: error = in6_selectsrc_socket(addr, inp->in6p_outputopts,
^ Wrapped

sys/netinet6/nd6_nbr.c: error = in6_selectsrc_addr(fibnum, &dst6,
^ Called in nd6_ns_output_fib().
It is used locally in nd6_dad_ns_output() which is called from nd6_dad_starttimer() which is already under epoch.
It is also used in nd6_llinfo_timer() and nd6_resolve_slow() - both are under epoch.
Given that, mark nd6_ns_output_fib () as requiring epoch by using NET_EPOCH_ASSERT.

sys/netinet6/nd6_nbr.c: error = in6_selectsrc_addr(fibnum, &dst6,
^ Called in nd6_na_output_fib().
It is used externally in carp_send_na() which already requires epoch.
It is used locally in nd6_ns_input(). The latter is used in icmp6_input() (already under epoch) and send_output() (added epoch calls).
Finally, nd6_na_output_fib() calls ip6_output() so it needs to be either under epoch or wrap ip6_output().
Given the above, mark nd6_na_output_fib() as requiring epoch.

sys/netinet6/icmp6.c: error = in6_selectsrc_addr(M_GETFIB(m), &dst6,
^^ Wrapped

sys/netinet6/in6_pcb.c: error = in6_selectsrc_socket(sin6, inp->in6p_outputopts,
^ Wrapped

sys/netinet6/udp6_usrreq.c: error = in6_selectsrc_socket(sin6, optp, inp,
^ already under epoch

[donner]

sys/netinet6/send.c

152

I assume, the other functions do not need epoch at the moment.
If they ever need it sometimes in the future, the scope can be extended to the whole switch.

[melifaro]

Address comments.