loopback: Implement underrun protection

loopback: Implement underrun protection

The latency controller will try to adjust to the configured latency regardless
of underruns. If the configured latency is set too small, it will lead to
periodically occuring underruns. Therefore an underrun protection is implemented
which will increase the target latency if too many underruns are detected.
Underruns are tracked and if more than 3 underruns occur, the target latency
is increased in increments of 5 ms. One underrun per hour is accepted.
The protection ensures, that independent from the configured latency the
module will converge to a stable latency if the configured latency is too
small.

The print_msg argument to update_minimum_latency() had to be re-introduced,
because there is one place where the message should not be logged.