nexthop: Emit a notification when a single nexthop is replaced

[ Upstream commit 8c09c9f9 ]

The notification is emitted after all the validation checks were
performed, but before the new configuration (i.e., 'struct nh_info') is
pointed at by the old shell (i.e., 'struct nexthop'). This prevents the
need to perform rollback in case the notification is vetoed.

The next patch will also emit a replace notification for all the nexthop
groups in which the nexthop is used.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: 390b3a30 ("nexthop: Forbid FDB status change while nexthop is in a group")
Signed-off-by: Sasha Levin <sashal@kernel.org>