rcu: Remove unused 'cpu' in rcu_virt_note_context_switch()

This commit removes the unused function argument 'cpu'.  This does not
change functionality, but might save a cycle or two.

Signed-off-by: Zeng Heng <zengheng4@huawei.com>
Acked-by: Mukesh Ojha <quic_mojha@quicinc.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>