mlxsw: spectrum_acl: Don't take rtnl lock during vregion_rehash_intrvl_set()

Relax dependency on rtnl mutex during vregion_rehash_intrvl_set(). The
vregion list is protected with newly introduced mutex.

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>