mlxsw: spectrum_router_xm: Implement L-value tracking for M-index

There is a table that assigns L-value per M-index. The L is always the
biggest from the currently inserted prefixes. Setup a hashtable to track
the M-index information and the prefixes that are related to it. Ensure
the L-value is always correctly set.

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