wakeup_source_add() adds an item into wakeup_sources list.

There is no need to call synchronize_rcu() at this point.

Its only needed in wakeup_source_remove()
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>