ublk: introduce and use ublk_set_canceling helper

ublk: introduce and use ublk_set_canceling helper

For performance reasons (minimizing the number of cache lines accessed
in the hot path), we store the "canceling" state redundantly - there is
one flag in the device, which can be considered the source of truth, and
per-queue copies of that flag. This redundancy can cause confusion, and
opens the door to bugs where the state is set inconsistently. Try to
guard against these bugs by introducing a ublk_set_canceling helper
which is the sole mutator of both the per-device and per-queue canceling
state. This helper always sets the state consistently. Use the helper in
all places where we need to modify the canceling state.

No functional changes are expected.

Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250703-ublk_too_many_quiesce-v2-2-3527b5339eeb@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>