ALSA: pcm: Kill set_fs() in PCM OSS layer

This is the last-standing one: kill the set_fs() usage in PCM OSS
layer by replacing with the new API functions to deal with the direct
in-kernel buffer copying.

The code to fill the silence can be replaced even to a one-liner to
pass NULL buffer instead of the manual copying.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>