rculist: Fix list_entry_rcu to read ptr with, rcu_dereference_raw

Hi Paul and Josh,

While running some benchmarks with rcu, I noticed that rculist had a performance issue compared to the non-rcu list. Having a closer look at the rlulist, I found that there is a bad usage of rcu_dereference_raw in list_entry_rcu(). Indeed, "typeof (*ptr) *__ptr = ptr;" reads ptr and "rcu_dereference_raw(__ptr)" forces read on the stack variable __ptr. However, ptr should be read with rcu_dereference_raw not the __ptr.

This was introduced some years ago with commit 67bdbffd (2010-02-25).
I am proposing to revert this part of the patch but probably this can be fixed differently.

Fixing this showed 2 problems in the usage of list_entry_rcu.
Since it touches other parts of kernel, I guess it requires ack from other maintainers.

Thanks.

PS: I will Cc the others maintainers when somebody confirms the problem.