ublk: don't issue uring_cmd from fallback task work

ublk: don't issue uring_cmd from fallback task work

When ublk_ch_uring_cmd_cb() runs as fallback task work (e.g., because
the submitting task is exiting), the command should not be issued as
current is a kworker, not the daemon task. This can cause io->task
to capture the wrong task in __ublk_fetch(), leading to a task
mismatch warning in ublk_uring_cmd_cancel_fn().

Check tw.cancel and return -ECANCELED instead of issuing the command
from fallback context.

Fixes: 3421c7f68bba ("ublk: make sure io cmd handled in submitter task context")
Signed-off-by: Ming Lei <tom.leiming@gmail.com>
Link: https://patch.msgid.link/20260501112312.947327-1-tom.leiming@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>