BACKPORT: FROMGIT: 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.

(cherry picked from commit 88147836e575d50b11d6519c7426540063fa892f
 https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git

 rcu/next)

(
Backport: Patch tree_plugin.h instead of tree_nocb.h,
	  register_shrinker changes,
	  changed to wake_nocb_gp call site (due to locking).
)

BUG=b:222463781
TEST=powerIdle lab tests.

Signed-off-by: Vineeth Pillai <vineeth@bitbyteword.org>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Joel Fernandes <joelaf@google.com>
Signed-off-by: Parshuram Sangle <parshuram.sangle@intel.corp-partner.google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/3965073


Reviewed-by: Ross Zwisler <zwisler@google.com>
Change-Id: I73923678c1ad74ca5fb6c5f727caf7f0d24b5a20
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4069542