ipv6: Unlink sibling route in case of failure

When a route needs to be appended to an existing multipath route,
fib6_add_rt2node() first appends it to the siblings list and increments
the number of sibling routes on each sibling.

Later, the function notifies the route via call_fib6_entry_notifiers().
In case the notification is vetoed, the route is not unlinked from the
siblings list, which can result in a use-after-free.

Fix this by unlinking the route from the siblings list before returning
an error.

Audited the rest of the call sites from which the FIB notification chain
is called and could not find more problems.

Fixes: 2233000c ("net/ipv6: Move call_fib6_entry_notifiers up for route adds")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Alexander Petrovskiy <alexpe@mellanox.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>