mlxsw: spectrum_kvdl: Protect allocations with a lock

The KVDL is used to store objects allocated throughout various places
in the driver. For example, both nexthops (adjacency entries) and ACL
actions are stored in the KVDL.

Currently, all allocations are protected by RTNL, but this is going to
change with the removal of RTNL from the routing code.

Therefore, protect KVDL allocations with a lock. A mutex is used since
the free operation can block in Spectrum-2.

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