ALSA: memalloc: Fix mmap of SG-buffer with WC pages

ALSA: memalloc: Fix mmap of SG-buffer with WC pages

The code refactoring to move the WC page handling into the common
memalloc layer caused a breakage for HD-audio HDMI.  I overlooked that
the driver is using the SG-buffer, which isn't covered by the patch.

This patch adds the mmap workaround for WC pages to SG-buffer
handler.  A caveat is that it falls back to the default handler by
returning an error after setting the pgprot, so it won't work in all
cases but merely for PCM (which is currently the only use case).

Fixes: 623c10108338 ("ALSA: memalloc: Fix pgprot for WC mmap on x86")
Reported-and-tested-by: Andy Lavr <andy.lavr@gmail.com>
Link: https://lore.kernel.org/r/20210808080034.20337-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>