Fix RCU list iterator use of 'rcu_dereference()'

The RCU iterators used 'rcu_dereference()' on an already-fetched RCU
pointer value, which defeats the whole point of the exercise.

When we dereference a pointer protected by RCU, we need to make sure
that we only fetch the value _once_, because if the compiler ends up
re-loading it due to register pressure, the newly reloaded value could
be different from the previously fetched one, and you get inconsistent
results.

Cleaned-up, fixed, and the pointless list_for_each_safe_rcu #define
deleted by Paul Kenney.

Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>