When poll request is timed out, it is removed from the poll list,
but not completed, so the request is leaked, and never get chance
to complete.

Fix the issue by ending it in timeout handler.

Fixes: 0a593fbb ("null_blk: poll queue support")
Signed-off-by: NMing Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220413084836.1571995-1-ming.lei@redhat.comSigned-off-by: NJens Axboe <axboe@kernel.dk>