rbd-nbd: Fix aio_callback error handling

Change aio_callback return code handling for NBD_CMD_READ command.

Tracker:
http://tracker.ceph.com/issues/15604

Signed-off-by: Chang-Yi Lee <cy.l@inwinstack.com>
(cherry picked from commit ee21129)

src/tools/rbd_nbd/rbd-nbd.cc

@@ -189,9 +189,10 @@ class NBDServer
     } else if ((ctx->command == NBD_CMD_READ) &&
                 ret < static_cast<int>(ctx->request.len)) {
       int pad_byte_count = static_cast<int> (ctx->request.len) - ret;
-      ctx->data.append('\x0', pad_byte_count);
+      ctx->data.append_zero(pad_byte_count);
       dout(20) << __func__ << ": " << *ctx << ": Pad byte count: " 
                << pad_byte_count << dendl;
+      ctx->reply.error = 0;
     } else {
       ctx->reply.error = htonl(0);
     }