Fix Driver Ext Tracing Legacy driver compatability

[nrspruit]

No description provided.

[rwmcguir]

Two small things.

pfnGet is called twice, but the first call's count is only used for the 0 == driverCount check — which the second call already reports. One call would do.

Bigger question: the loader keys extension registration and fan-out per handle (ExtensionFunctionKey{hDriver, functionName} in tracing_imp.h), but this gates per library using one cached handle. That's fine if zelDriverEnableTracing is defined as library-global — all handles in a driver share a code base, so one handle can speak for all of them. Could the header say that explicitly? Otherwise a driver that keeps the flag per handle silently traces only the first one.