Merge branch 'ipv6_ecmp_fixes'

Michal Kubecek says:

====================
IPv6 ECMP route add/replace fixes

(1) When adding a nexthop of a multipath route fails (e.g. because of a
conflict with an existing route), we are supposed to delete nexthops
already added. However, currently we try to also delete all nexthops we
haven't even tried to add yet so that a "ip route add" command can
actually remove pre-existing routes if it fails.

(2) Attempt to replace a multipath route results in a broken siblings
linked list. Following commands (like "ip route del") can then either
follow a link into freed memory or end in an infinite loop (if the slab
object has been reused).

v2: fix an omission in first patch

v3: change the semantics of replace operation to better match IPv4
====================

Signed-off-by: David S. Miller <davem@davemloft.net>