[log] check log level before dumping packet to string (#7138)

`otDump` will currently always dump to string and use the log level to
decide whether to print it. Since this function is in the key path of
packet handling, this behavior reduces network throughput. This commit
adds a log level check to fix the issue.

src/core/common/logging.cpp

@@ -297,6 +297,8 @@ void otDump(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aId, const
     size_t                        idLen = strlen(aId) + kFixedStringPart;
     ot::String<kStringLineLength> string;
 
+    VerifyOrExit(otLoggingGetLevel() >= aLogLevel);
+
     for (size_t i = 0; i < (kWidth - idLen) / 2; i++)
     {
         string.Append("=");
@@ -325,6 +327,9 @@ void otDump(otLogLevel aLogLevel, otLogRegion aLogRegion, const char *aId, const
     }
 
     otLogDump(aLogLevel, aLogRegion, "%s", string.AsCString());
+
+exit:
+    return;
 }
 #else  // OPENTHREAD_CONFIG_LOG_PKT_DUMP
 void otDump(otLogLevel, otLogRegion, const char *, const void *, const size_t)