Input: use guard notation in input core

Switch input core to use "guard" notation when acquiring spinlocks and
mutexes to simplify the code and ensure that locks are automatically
released when control leaves critical section.

Link: https://lore.kernel.org/r/20241107071538.195340-9-dmitry.torokhov@gmail.com


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