mlxsw: spectrum_acl: Introduce a mutex to guard objagg instance manipulation

Protect objagg structures by adding a mutex to ERP code and take it
during the structure manipulation.

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