locktorture: Ignore nreaders_stress if no readlock support

Exclusive locks do not have readlock support, which means that a
locktorture run with the following module parameters will do nothing:

 torture_type=mutex_lock nwriters_stress=0 nreaders_stress=1

This commit therefore rejects this combination for exclusive locks by
returning -EINVAL during module init.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>