mlxsw: spectrum_router: Share nexthop counters in resilient groups

For resilient groups, we can reuse the same counter for all the buckets
that share the same nexthop. Keep a reference count per counter, and keep
all these counters in a per-next hop group xarray, which serves as a
NHID->counter cache. If a counter is already present for a given NHID, just
take a reference and use the same counter.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://lore.kernel.org/r/cdd00084533fc83ac5917562f54642f008205bf3.1709901020.git.petrm@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>