ALSA: pcm: fix playback silence - correct incremental silencing

ALSA: pcm: fix playback silence - correct incremental silencing

Commit 9a826ddba6e ("[ALSA] pcm core: fix silence_start calculations")
came with exactly the right commit message, but the patch just made
things broken in a different way: We'd fill at a too low address if the
area was already partially zeroed, so we'd under-fill. This affected
both thresholded mode (where it was somewhat less likely) and top-up
mode (where it would be the case consistently).

Co-developed-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Oswald Buddenhagen <oswald.buddenhagen@gmx.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>
Link: https://lore.kernel.org/r/20230505155244.2312199-3-oswald.buddenhagen@gmx.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>