loopback: Fix sink latency calculation if source is alsa source with fixed latency

loopback: Fix sink latency calculation if source is alsa source with fixed latency

When an alsa source with fixed latency is used, the actual latency of the source
will only be one fragment size. This is not taken into account when the required
sink latency is calculated.

This patch fixes the issue.

[jungsup4.lee: backport mainline commit 0efae0]

[Version] 13.0-33
[Issue Type] Bug fix

Change-Id: If8b7aa3735c1a2456fa50f55b267180fa2efd1a1
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/451>