BACKPORT: rcu: Shrinker for lazy rcu

The shrinker is used to speed up the free'ing of memory potentially held
by RCU lazy callbacks. RCU kernel module test cases show this to be
effective. Test is introduced in a later patch.

[Joel: register_shrinker() argument list change.]

Bug: 258241771
Bug: 222463781
Test: CQ
Change-Id: I6a73a9dae79ff35feca37abe2663e55a0f46dda8
Signed-off-by: Vineeth Pillai <vineeth@bitbyteword.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
(cherry picked from commit c945b4da)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4318046


Tested-by: Joel Fernandes <joelaf@google.com>
Reviewed-by: Vineeth Pillai <vineethrp@google.com>
Commit-Queue: Joel Fernandes <joelaf@google.com>
(cherry picked from commit 2cf50ca2e7c3bc08f5182fc517a89a65e8dca7e3)
[Cherry picked from chromeos-5.15 tree. Minor tweaks to commit message
to match Android style]
Signed-off-by: Qais Yousef <qyousef@google.com>