f_lavfi: add gross workaround for af_dynaudnorm bug

Better do this here than deal with the moronic project we unfortunately
depend on.

The workaround is generic; unknown whether it works correctly with
multi-input/output filters or filter graphs. It assumes that if all
inputs are EOF, and all outputs are EAGAIN, the bug happened.

This is pretty tricky, because anything could happen. Any time some form
of progress is made, the got_eagain state needs to be reset, because the
filter pad's state could have changed.