block: mirror - change string allocation to 2-bytes

The backing_filename string in mirror_run() is only used to check
for a NULL string, so we don't need to allocate 1024 bytes (or, later,
PATH_MAX bytes), when we only need to copy the first 2 characters.

We technically only need 1 byte, as we are just checking for NULL, but
since backing_filename[] is populated by bdrv_get_backing_filename(), a
string size of 1 will always only return '\0';

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/mirror.c

@@ -378,7 +378,8 @@ static void coroutine_fn mirror_run(void *opaque)
     int64_t sector_num, end, sectors_per_chunk, length;
     uint64_t last_pause_ns;
     BlockDriverInfo bdi;
-    char backing_filename[1024];
+    char backing_filename[2]; /* we only need 2 characters because we are only
+                                 checking for a NULL string */
     int ret = 0;
     int n;