torture: Add a kthread-creation callback to _torture_create_kthread()

[ Upstream commit 67d5404d ]

This commit adds a kthread-creation callback to the
_torture_create_kthread() function, which allows callers of a new
torture_create_kthread_cb() macro to specify a function to be invoked
after the kthread is created but before it is awakened for the first time.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Valentin Schneider <vschneid@redhat.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: kernel-team@android.com
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: John Stultz <jstultz@google.com>
Stable-dep-of: cca42bd8eb1b ("rcutorture: Fix stuttering races and other issues")
Signed-off-by: Sasha Levin <sashal@kernel.org>