netinet6: Remove ndpr_raf_ra_derived flag

[hrs]

Summary

This flag was introduced at 8036234c72c9361711e867cc1a0c6a7fe0babd84 to
prevent the SIOCSPFXFLUSH_IN6 ioctl from removing manually-added entries.
However, this flag did not work well with transitions such as that
the same prefix was manually configured before or after receiving RAs.
Adding an alias manually to one using SLAAC is a typical configuration.

This patch removes the flag because a prefix is derived from an RA
always has an entry in the ndpr_advrtrs member in the struct nd_prefix
or in NDPRF_DETACHED state with no router after the node is attached
to another network. Having a separate flag is not a good idea
because it can cause a mismatch between the flag and the actual situation.

This also removes a prefix check in the ICMPV6CTL_ND6_PRLIST sysctl to
exclude manually-added entries. This ioctl is designed to list all entries,
and there is no relationship to SIOCSPFXFLUSH_IN6.

[hrs]

@melifaro Could you explain the motivation for excluding manually-added prefixes from ICMPV6CTL_ND6_PRLIST? I could not find the reason in the review comments for the original change. As I wrote in the description, this sysctl is to list all prefixes in kernel. If a use case needs to pick up a subset of prefixes, it should be handled after getting the whole list.