When hw_stats is set on a group, install nexthop counters on members of a
group.

Counter allocation request is moved from nexthop object initialization to
the update code. The previous placement made sense: when the counters are
enabled by dpipe, the counters are installed to all existing nexthops and
all nexthops created from then on get them. For the finer-grained nexthop
group statistics, this is unsuitable. The existing placement was kept for
the IPv4 and IPv6 nexthops.

Resilient group replacement emits a pre_replace notification, and then any
bucket_replace notifications if there were any replacements at all. If the
group is balanced and the nexthop composition of the replaced group didn't
change, there will be no such notifiers. Therefore hook to the pre_replace
notifier and mark all buckets for update, to un/install the counters.

When reporting deltas for resilient groups, use the nexthop ID that we
stored in a previous patch to look up to which nexthop a bucket
contributes.
Co-developed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/87495a72f187df2e5d491d02729c550d235fcc85.1709901020.git.petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>