ALSA: memalloc: Use proper SG helpers for noncontig allocations

The recently introduced non-contiguous page allocation support helpers
are using the simplified code to calculate the page and DMA address
based on the vmalloc helpers, but this isn't quite right as the vmap
is valid only for the direct DMA.

This patch corrects those accessors to use the proper SG helpers
instead.

Fixes: a25684a9

 ("ALSA: memalloc: Support for non-contiguous page allocation")
Tested-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
Link: https://lore.kernel.org/r/20211108151059.31898-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>