92a9f11a0b added locking for dec_sub.
At that time, because the lock was exposed to the outside world,
a recursive mutex was used. However, this is no longer true after
e9e883e3b2, when the public locking
functions were removed. This means that the lock is now private.
Unlike input.c, dec_sub already enforces said call hierarchy, so
combined with the aforementioned change, the lock is only ever
called once and never recursively. Thus the lock can be converted to
a normal mutex.
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