Use abstracted RCU API to dereference RCU protected data.  Hides barrier
details.  Patch from Paul McKenney.

This patch introduced an rcu_dereference() macro that replaces most uses of
smp_read_barrier_depends().  The new macro has the advantage of explicitly
documenting which pointers are protected by RCU -- in contrast, it is
sometimes difficult to figure out which pointer is being protected by a given
smp_read_barrier_depends() call.
Signed-off-by: Paul McKenney <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>