mlxsw: spectrum_mr: Protect multicast table list with a lock

mlxsw: spectrum_mr: Protect multicast table list with a lock

The multicast table list is traversed from a delayed work that
periodically updates the kernel about packets and bytes statistics from
each multicast route.

The list is currently protected by RTNL, but subsequent patches will
remove the driver's dependence on this contended lock.

In order to be able to remove dependence on RTNL in the next patch,
guard this list with a dedicated mutex.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>