ublk: store request pointer in ublk_io

ublk: store request pointer in ublk_io

A ublk_io is converted to a request in several places in the I/O path by
using blk_mq_tag_to_rq() to look up the (qid, tag) on the ublk device's
tagset. This involves a bunch of dereferences and a tag bounds check.

To make this conversion cheaper, store the request pointer in ublk_io.
Overlap this storage with the io_uring_cmd pointer. This is safe because
the io_uring_cmd pointer is only valid if UBLK_IO_FLAG_ACTIVE is set on
the ublk_io, the request pointer is valid if UBLK_IO_FLAG_OWNED_BY_SRV,
and these flags are mutually exclusive.

Suggested-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250430225234.2676781-10-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>