srcutiny: Make a Tiny SRCU grace period imply an RCU grace period

In non-preemptible kernels, a Tiny SRCU grace period implies an RCU
grace period because any context switch suffices.  But in preemptible
kernels, it is possible for a Tiny SRCU grace period to elapse without
a corresponding RCU grace period.

Which was OK until RCU Tasks Trace was re-implemented in terms
of SRCU-fast, which in Tiny SRCU is implemented as SRCU, which is
already fast.  And RCU Tasks Trace grace periods are required to imply
RCU grace periods.  This commit therefore adds a synchronize_rcu(),
but only in preemptible kernels.

Because preemptible Tiny SRCU is not in mainline, this added call to
synchronize_rcu() will not slow anything down:  The comparison would
instead be with TREE SRCU.  But if this added call ever becomes a problem,
the Tiny SRCU srcu_struct structure could track whether or not this is for
SRCU-fast, and to add the synchronize_rcu() only in the SRCU-fast case.
However, at the moment, this is seen as unnecessary complexity.

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

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