Input: poller - convert locking to guard notation

Use guard() notation instead of explicitly acquiring and releasing
mutex to simplify the code and ensure that it is released.

Link: https://lore.kernel.org/r/20241107071538.195340-8-dmitry.torokhov@gmail.com


Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>