block: null_blk: end timed out poll request

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: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20220413084836.1571995-1-ming.lei@redhat.com


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