ublk: restore auto buf unregister refcount optimization

Commit 1ceeedb59749 ("ublk: optimize UBLK_IO_UNREGISTER_IO_BUF on daemon
task") optimized ublk request buffer unregistration to use a non-atomic
reference count decrement when performed on the ublk_io's daemon task.
The optimization applied to auto buffer unregistration, which happens as
part of handling UBLK_IO_COMMIT_AND_FETCH_REQ on the daemon task.
However, commit b749965edda8 ("ublk: remove ublk_commit_and_fetch()")
reordered the ublk_sub_req_ref() for the completed request before the
io_buffer_unregister_bvec() call. As a result, task_registered_buffers
is already 0 when io_buffer_unregister_bvec() calls ublk_io_release()
and the non-atomic refcount optimization doesn't apply.
Move the io_buffer_unregister_bvec() call back to before
ublk_need_complete_req() to restore the reference counting optimization.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Fixes: b749965edda8 ("ublk: remove ublk_commit_and_fetch()")
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>