rcutorture: Warn on individual rcu_torture_init() error conditions

rcutorture: Warn on individual rcu_torture_init() error conditions

[ Upstream commit efeff6b39b9de4480572c7b0c5eb77204795cb57 ]

When running rcutorture as a module, any rcu_torture_init() issues will be
reflected in the error code from modprobe or insmod, as the case may be.
However, these error codes are not available when running rcutorture
built-in, for example, when using the kvm.sh script.  This commit
therefore adds WARN_ON_ONCE() to allow distinguishing rcu_torture_init()
errors when running rcutorture built-in.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>