[fuzz] limit PBKDF2 iterations to avoid OSS-Fuzz timeout (#5069)

src/core/crypto/pbkdf2_cmac.cpp

@@ -63,6 +63,11 @@ void otPbkdf2Cmac(const uint8_t *aPassword,
     OT_ASSERT(aIterationCounter % 2 == 0);
     aIterationCounter /= 2;
 
+#if FUZZING_BUILD_MODE_UNSAFE_FOR_PRODUCTION
+    // limit iterations to avoid OSS-Fuzz timeouts
+    aIterationCounter = 2;
+#endif
+
     while (keyLen)
     {
         ++blockCounter;