ublk: don't call ublk_dispatch_req() for NEED_GET_DATA

ublk: don't call ublk_dispatch_req() for NEED_GET_DATA

ublk_dispatch_req() currently handles 3 different cases: incoming ublk
requests that don't need to wait for a data buffer, incoming requests
that do need to wait for a buffer, and resuming those requests once the
buffer is provided. But the call site that provides a data buffer
(UBLK_IO_NEED_GET_DATA) is separate from those for incoming requests.

So simplify the function by splitting the UBLK_IO_NEED_GET_DATA case
into its own function ublk_get_data(). This avoids several redundant
checks in the UBLK_IO_NEED_GET_DATA case, and streamlines the incoming
request cases.

Don't call ublk_fill_io_cmd() for UBLK_IO_NEED_GET_DATA, as it's no
longer necessary to set io->cmd or the UBLK_IO_FLAG_ACTIVE flag for
ublk_dispatch_req().

Since UBLK_IO_NEED_GET_DATA no longer relies on ublk_dispatch_req()
calling io_uring_cmd_done(), return the UBLK_IO_RES_OK status directly
from the ->uring_cmd() handler. If ublk_start_io() fails, don't complete
the UBLK_IO_NEED_GET_DATA command, matching the existing behavior.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250430225234.2676781-8-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>