ublk: specify io_cmd_buf pointer type

ublk: specify io_cmd_buf pointer type

io_cmd_buf points to an array of ublksrv_io_desc structs but its type is
char *. Indexing the array requires an explicit multiplication and cast.
The compiler also can't check the pointer types.

Change io_cmd_buf's type to struct ublksrv_io_desc * so it can be
indexed directly and the compiler can type-check the code.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Link: https://lore.kernel.org/r/20250328194230.2726862-2-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>