In commit 4b1373de ("net: ipv6: addr: perform strict checks also for
doit handlers") we add strict check for inet6_rtm_getaddr(). But we did
the invalid header values check before checking if NETLINK_F_STRICT_CHK
is set. This may break backwards compatibility if user already set the
ifm->ifa_prefixlen, ifm->ifa_flags, ifm->ifa_scope in their netlink code.

I didn't move the nlmsg_len check because I thought it's a valid check.
Reported-by: NJianlin Shi <jishi@redhat.com>
Fixes: 4b1373de ("net: ipv6: addr: perform strict checks also for doit handlers")
Signed-off-by: NHangbin Liu <liuhangbin@gmail.com>
Reviewed-by: NDavid Ahern <dsahern@gmail.com>
Signed-off-by: NJakub Kicinski <jakub.kicinski@netronome.com>