srcu-3: add RCU variant that permits read-side blocking

Version 3 of the SRCU patchset.

This patch incorporates a number of improvements, many of which came
up in off-list discussions with Alan Stern.  Neither of us are sure
why these discussions ended up off-list, so I have summarized them
below.

o	Fixes some "zombie code" -- excess curly braces and the like.

o	Gets rid of the double-flip in favor of an additional
	synchronize_sched().  This turned out to be safe, despite
	my saying otherwise at http://lkml.org/lkml/2006/6/27/486.
	The trick that I was missing is that synchronize_sched()
	forces all CPUs to execute at least one memory barrier during
	the synchronize_sched()'s execution, which forces all CPUs
	to see synchronize_srcu()'s counter increment as happening
	after any memory manipulations prior to the synchronize_srcu().

	Upgraded comments to indicate what the synchronize_sched()
	calls are needed for.

o	Added a barrier() to both srcu_read_lock() and srcu_read_unlock()
	to prevent the compiler from performing optimizations that
	would cause the critical section to move outside of the
	enclosing srcu_read_lock() and srcu_read_unlock().

	However, these barrier()s in srcu_read_lock() and srcu_read_unlock()
	are needed only in non-CONFIG_PREEMPT kernels, so they compile
	to nothing in CONFIG_PREEMPT kernels (where the preempt_disable()
	and preempt_enable() calls supply the needed barrier() call).

o	Added a check to synchronize_srcu() that permits this primitive
	to take advantage of grace periods induced by concurrent executions
	in other tasks.  This can be useful in cases where you are
	using a single srcu_struct to handle all the individually-locked
	chains of a hash table, for example.

o	cleanup_srcu_struct() now contains error checks to catch cases
	where readers are still using the srcu_struct in question.
	It does a WARN_ON() and leaks the srcu_struct's per-CPU data
	in that case.

o	There is an srcu_readers_active() that returns the number of
	readers (approximate!) currently using the specified srcu_struct.
	This can be useful when terminating use of an srcu_struct, e.g.,
	at module-unload time.

o	Improved the RCU torture tests, increasing the skew on reader
	times and providing implementation-specific delay functions.