Changes

io_uring: Avoid extra reads if iofd becomes unregistered by user

The user callback (read_cb, segmentation_cb, etc.) may decide to
unregister the iofd instead of closing it. In this scenario, we should
avoid calling the callback further until the iofd is registered again.

This is a backport of master branch commit
32efc324ce0b30280effef4860bb15588ecb1010.

Change-Id: I514412ecfbb82d32cb3d9c2272795ebac42f55e9