mlxsw: spectrum_router: Update adjacency index more efficiently

The device supports an operation that allows the driver to issue one
request to update the adjacency index for all the routes in a given
virtual router (VR) from old index and size to new ones. This is useful
in case the configuration of a certain nexthop group is updated and its
adjacency index changes.

Currently, the driver does not use this operation in an efficient
manner. It iterates over all the routes using the nexthop group and
issues an update request for the VR if it is not the same as the
previous VR.

Instead, use the VR tracking added in the previous patch to update the
adjacency index once for each VR currently using the nexthop group.

Example:

8k IPv6 routes were added in an alternating manner to two VRFs. All the
routes are using the same nexthop object ('nhid 1').

Before:

# perf stat -e devlink:devlink_hwmsg --filter='incoming==0' -- ip nexthop replace id 1 via 2001:db8:1::2 dev swp3

 Performance counter stats for 'ip nexthop replace id 1 via 2001:db8:1::2 dev swp3':

            16,385      devlink:devlink_hwmsg

       4.255933213 seconds time elapsed

       0.000000000 seconds user
       0.666923000 seconds sys

Number of EMAD transactions corresponds to number of routes using the
nexthop group.

After:

# perf stat -e devlink:devlink_hwmsg --filter='incoming==0' -- ip nexthop replace id 1 via 2001:db8:1::2 dev swp3

 Performance counter stats for 'ip nexthop replace id 1 via 2001:db8:1::2 dev swp3':

                 3      devlink:devlink_hwmsg

       0.077655094 seconds time elapsed

       0.000000000 seconds user
       0.076698000 seconds sys

Number of EMAD transactions corresponds to number of VRFs / VRs.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>