block/io: bdrv_wait_serialising_requests_locked: drop extra bs arg

bs is linked in req, so no needs to pass it separately. Most of
tracked-requests API doesn't have bs argument. Actually, after this
patch only tracked_request_begin has it, but it's for purpose.

While being here, also add a comment about what "_locked" is.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20201021145859.11201-5-vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>