block: Assert serialisation assumptions in pwritev

If a request calls wait_serialising_requests() and actually has to wait
in this function (i.e. a coroutine yield), other requests can run and
previously read data (like the head or tail buffer) could become
outdated. In this case, we would have to restart from the beginning to
read in the updated data.

However, we're lucky and don't actually need to do that: A request can
only wait in the first call of wait_serialising_requests() because we
mark it as serialising before that call, so any later requests would
wait. So as we don't wait in practice, we don't have to reload the data.

This is an important assumption that may not be broken or data
corruption will happen. Document it with some assertions.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
