UPSTREAM: rcu: Update synchronize_rcu_mult() comment for call_rcu_hurry()

Those who have worked with RCU for some time will naturally think in
terms of the long-standing call_rcu() API rather than the much newer
call_rcu_hurry() API.  But it is call_rcu_hurry() that you should normally
pass to synchronize_rcu_mult().  This commit therefore updates the header
comment to point this out.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
(cherry picked from commit 6716f4d3)

Bug: 258241771
Signed-off-by: Joel Fernandes <joelaf@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/4909037


Reviewed-by: Ross Zwisler <zwisler@google.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Reviewed-by: Vineeth Pillai <vineethrp@google.com>
Signed-off-by: Qais Yousef <qyousef@google.com>
Change-Id: I0d701825ddd7e15cebd92190388fbf78c04d26fb