block: Move read-only check during truncation earlier

No need to start a tracked request that will always fail.  The choice
to check read-only after bdrv_inc_in_flight() predates 1bc5f09f
(block: Use tracked request for truncate), but waiting for serializing
requests can make the effect more noticeable.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20210609163034.997943-1-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block/io.c

@@ -3392,6 +3392,11 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
         return old_size;
     }
 
+    if (bdrv_is_read_only(bs)) {
+        error_setg(errp, "Image is read-only");
+        return -EACCES;
+    }
+
     if (offset > old_size) {
         new_bytes = offset - old_size;
     } else {
@@ -3408,11 +3413,6 @@ int coroutine_fn bdrv_co_truncate(BdrvChild *child, int64_t offset, bool exact,
     if (new_bytes) {
         bdrv_make_request_serialising(&req, 1);
     }
-    if (bdrv_is_read_only(bs)) {
-        error_setg(errp, "Image is read-only");
-        ret = -EACCES;
-        goto out;
-    }
     ret = bdrv_co_write_req_prepare(child, offset - new_bytes, new_bytes, &req,
                                     0);
     if (ret < 0) {