ublk: add `union ublk_io_buf` with improved naming

ublk: add `union ublk_io_buf` with improved naming

Add `union ublk_io_buf` for naming the anonymous union of struct ublk_io's
addr and buf fields, meantime apply it to `struct ublk_io` for storing either
ublk auto buffer register data or ublk server io buffer address.

The union uses clear field names:
- `addr`: for regular ublk server io buffer addresses
- `auto_reg`: for ublk auto buffer registration data

This eliminates confusing access patterns and improves code readability.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>