[nat64-translator] ensure expired mappings are released in timer callback (openthread#11139)

This commit fixes the release of expired mappings by ensuring that
`ReleaseExpiredMappings()` is always called from the timer callback
`HandleMappingExpirerTimer()`, regardless of the logging level.
Previously, `ReleaseExpiredMappings()` was called as an input to
`LogInfo()`, which could become an empty macro (ignoring its input)
if info-level logs were disabled.

Author: abtink

src/core/net/nat64_translator.cpp

@@ -552,8 +552,13 @@ void Translator::ClearNat64Prefix(void)
 
 void Translator::HandleMappingExpirerTimer(void)
 {
-    LogInfo("Released %d expired mappings", ReleaseExpiredMappings());
+    uint16_t numReleased = ReleaseExpiredMappings();
+
+    LogInfo("Released %u expired mappings", numReleased);
+
     mMappingExpirerTimer.Start(kAddressMappingIdleTimeoutMsec);
+
+    OT_UNUSED_VARIABLE(numReleased);
 }
 
 void Translator::InitAddressMappingIterator(AddressMappingIterator &aIterator)