DataFlash: reset write log on new log

libraries/DataFlash/DataFlash_File.cpp

@@ -383,6 +383,8 @@ uint16_t DataFlash_File::start_new_log(void)
         return 0xFFFF;
     }
     _write_offset = 0;
+    _writebuf_head = 0;
+    _writebuf_tail = 0;
     log_write_started = true;
 
     // now update lastlog.txt with the new log number

libraries/DataFlash/DataFlash_File.h

@@ -15,6 +15,7 @@
 #else
 #define perf_begin(x)
 #define perf_end(x)
+#define perf_count(x)
 #endif