rcu: Remove unneeded __rcu_process_callbacks() argument

With the advent of __this_cpu_ptr(), it is no longer necessary to pass
both the rcu_state and rcu_data structures into __rcu_process_callbacks().
This commit therefore computes the rcu_data pointer from the rcu_state
pointer within __rcu_process_callbacks() so that callers can pass in
only the pointer to the rcu_state structure.  This paves the way for
linking the rcu_state structures together and iterating over them.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>