nbd/server: fix NBD_CMD_CACHE

We should not go to structured-read branch on CACHE command, fix that.

Bug introduced in bc37b06a "nbd/server: introduce NBD_CMD_CACHE"
with the whole feature and affects 3.0.0 release.
Signed-off-by: NVladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
CC: qemu-stable@nongnu.org
Message-Id: <20181003144738.70670-1-vsementsov@virtuozzo.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
[eblake: commit message typo fix]
Signed-off-by: NEric Blake <eblake@redhat.com>

nbd/server.c

@@ -2177,7 +2177,8 @@ static coroutine_fn int nbd_do_cmd_read(NBDClient *client, NBDRequest *request,
     }
 
     if (client->structured_reply && !(request->flags & NBD_CMD_FLAG_DF) &&
-        request->len) {
+        request->len && request->type != NBD_CMD_CACHE)
+    {
         return nbd_co_send_sparse_read(client, request->handle, request->from,
                                        data, request->len, errp);
     }