srcu: Create concurrency-safe helper for initiating size transition

srcu: Create concurrency-safe helper for initiating size transition

Once there are contention-initiated size transitions, it will be
possible for rcutorture to initiate a transition at the same time
as a contention-initiated transition.  This commit therefore creates
a concurrency-safe helper function named srcu_transition_to_big() to
safely initiate size transitions.

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