rcu/kvfree: Move bulk/list reclaim to separate functions

The kvfree_rcu() code maintains lists of pages of pointers, but also a
singly linked list, with the latter being used when memory allocation
fails.  Traversal of these two types of lists is currently open coded.
This commit simplifies the code by providing kvfree_rcu_bulk() and
kvfree_rcu_list() functions, respectively, to traverse these two types
of lists.  This patch does not introduce any functional change.

Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>