ublk: look up ublk task via its pid in timeout handler

Look up ublk process via its pid in timeout handler, so we can avoid to
touch io->task, because it is fragile to touch task structure.

It is fine to kill ublk server process and this way is simpler.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-3-ming.lei@redhat.com


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