All evdev clients share a common waitgroup. On new input events, all
clients waiting on this waitgroup are woken up, even those filtering out
the events, possibly more than once per event. This leads to duplicated
and unwanted wakeups.

Split the shared waitgroup into per-client waitgroups for more
fine-grained wakeups.
Signed-off-by: Kenny Levinsen <kl@kl.wtf>
Link: https://lore.kernel.org/r/20200429184126.2155-1-kl@kl.wtfSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>