rcuscale: Provide clear error when async specified without primitives

Currently, if the rcuscale module's async module parameter is specified
for RCU implementations that do not have sync primitives such as
call_rcu(), there will be a series of splats due to calls to a NULL
pointer.  This commit therefore warns of this situation, but switches
to non-async testing.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
1 file changed