UPSTREAM: rcu/kfree: Fix kfree_rcu_shrink_count() return value

As per the comments in include/linux/shrinker.h, .count_objects callback
should return the number of freeable items, but if there are no objects
to free, SHRINK_EMPTY should be returned. The only time 0 is returned
should be when we are unable to determine the number of objects, or the
cache should be skipped for another reason.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
(cherry picked from commit 38269096)

Bug: 258241771
Bug: 222463781
Test: CQ
Change-Id: I5cb380fceaccc85971a47773d9058f0ea044c6dd
Signed-off-by: Joel Fernandes <joelaf@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4332178


Reviewed-by: Vineeth Pillai <vineethrp@google.com>
Reviewed-by: Sean Paul <sean@poorly.run>
(cherry picked from commit 3243f1e22bf915c9b805a96cc4a8cbc03ed5d7a8)
[Cherry picked from chromeos-5.15 tree. Minor tweaks to commit message
to match Android style]
Signed-off-by: Qais Yousef <qyousef@google.com>