FROMGIT: rcu/sync: Use call_rcu_flush() instead of call_rcu

call_rcu() changes to save power will slow down rcu sync. Use the
call_rcu_flush() API instead which reverts to the old behavior.

(cherry picked from commit 438855f961dd8f7b1350a5d533ff7dc7adc79fb5
 https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git

 rcu/next)

BUG=b:222463781
TEST=powerIdle lab tests.

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/+/3965075


Reviewed-by: Ross Zwisler <zwisler@google.com>
Change-Id: Ibecdb2c54b2978d856425a912b5107d4fd144512
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4069544