eventfd: release mutex before calling poll_wake()

Nadav pointed out that calling poll_wake() with the mutex taken may cause
unnecessary context switches, since the woken thread will also want to
acquire that mutex.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Paweł Dziepak <pdziepak@quarnos.org>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>