torture: Add get_torture_init_jiffies() for test-start time

This commit adds a get_torture_init_jiffies() function that returns the
value of the jiffies counter at the start of the test, that is, at the
point where torture_init_begin() was invoked.

This will be used to enable torture-test holdoffs for tests implemented
using per-CPU kthreads, which are created and deleted by CPU-hotplug
operations, and thus (unlike normal kthreads) don't automatically know
when the test started.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>