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.

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)

Conflicts:
   kernel/rcu/tree_nocb.h

Trivial conflict due to: "rcu/nocb: Add an option to offload all CPUs on boot"

Bug: 258241771
Signed-off-by: Joel Fernandes <joelaf@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4909032


Reviewed-by: Vineeth Pillai <vineethrp@google.com>
Signed-off-by: Qais Yousef <qyousef@google.com>
Change-Id: I6a73a9dae79ff35feca37abe2663e55a0f46dda8