ublk: factor out ublk_commit_and_fetch

ublk: factor out ublk_commit_and_fetch

Move the logic for the UBLK_IO_COMMIT_AND_FETCH_REQ opcode into its own
function. This also allows us to mark ublk_queue pointers as const for
that operation, which can help prevent data races since we may allow
concurrent operation on one ublk_queue in the future. Also open code
ublk_commit_completion in ublk_commit_and_fetch to reduce the number of
parameters/avoid a redundant lookup.

[Restore __ublk_ch_uring_cmd() req variable used in commit d6aa0c178bf8
("ublk: call ublk_dispatch_req() for handling UBLK_U_IO_NEED_GET_DATA")]

Suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Uday Shankar <ushankar@purestorage.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250430225234.2676781-2-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>