Merge branch 'ipv6-f6i-fib6_siblings-and-rt-fib6_nsiblings-fixes'

Eric Dumazet says:

====================
ipv6: f6i->fib6_siblings and rt->fib6_nsiblings fixes

Series based on an internal syzbot report with a repro.

After fixing (in the first patch) the original minor issue,
I found that syzbot repro was able to trigger a second
more serious bug in rt6_nlmsg_size().

Code review then led to the two final patches.

I have not released the syzbot bug, because other issues
still need investigations.
====================

Link: https://patch.msgid.link/20250725140725.3626540-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>