Input: ff-core - convert locking to guard notation

Use guard() and scoped_guard() notation instead of explicitly acquiring
and releasing spinlocks and mutexes to simplify the code and ensure that
all locks are released properly.

Link: https://lore.kernel.org/r/20241107071538.195340-2-dmitry.torokhov@gmail.com


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