io_uring: don't sleep when polling for I/O

There is no point in sleeping for the expected I/O completion timeout
in the io_uring async polling model as we never poll for a specific
I/O.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Mark Wunderlich <mark.wunderlich@intel.com>
Link: https://lore.kernel.org/r/20211012111226.760968-11-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>