ublk: optimize UBLK_IO_REGISTER_IO_BUF on daemon task

ublk: optimize UBLK_IO_REGISTER_IO_BUF on daemon task

ublk_register_io_buf() performs an expensive atomic refcount increment,
as well as a lot of pointer chasing to look up the struct request.

Create a separate ublk_daemon_register_io_buf() for the daemon task to
call. Initialize ublk_io's reference count to a large number, introduce
a field task_registered_buffers to count the buffers registered on the
daemon task, and atomically subtract the large number minus
task_registered_buffers in ublk_commit_and_fetch().

Also obtain the struct request directly from ublk_io's req field instead
of looking it up on the tagset.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250620151008.3976463-12-csander@purestorage.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>