mlxsw: spectrum: Protect counter pool with a lock

The counter pool is a shared resource. It is used by both the ACL code
to allocate counters for actions and by the routing code to allocate
counters for adjacency entries (for example).

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

Therefore, protect counter allocations with a spin lock.

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