mlxsw: spectrum_acl: Introduce a mutex to guard region list updates

In order to remove RTNL lock dependency, it is needed to protect
the regions list in a group. Introduce a mutex to do the job.

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>