Varnish uses a global per-probe timeout for backend probes. When reading
the backend response, Varnish tries to poll and read in a loop, until a
poll timeouts, the streams EOFs or there is an error.

The poll is supposed to timeout when the per-probe timeout ends. This is
currently setup so that `t_end` is the deadline for the probe, set when
the function starts, then the poll waits until `t_end`.

Previously, the poll timeout was never updated, and was always set to
`t_end - t_now` without updating `t_now`, which means that it was
effectively a between-bytes timeout instead of a proper per-probe
timeout.

This fixes this issue by updating `t_now` before updating the
`t_end - t_now` timeout so that the timeout passed to poll effectively
corresponds to a deadline of `t_end`.

See the issue fixed by this commit for more details.

Fixes: #3402