ALSA: memalloc: Workaround for Xen PV

We change recently the memalloc helper to use
dma_alloc_noncontiguous() and the fallback to get_pages().  Although
lots of issues with IOMMU (or non-IOMMU) have been addressed, but
there seems still a regression on Xen PV.  Interestingly, the only
proper way to work is use dma_alloc_coherent().  The use of
dma_alloc_coherent() for SG buffer was dropped as it's problematic on
IOMMU systems.  OTOH, Xen PV has a different way, and it's fine to use
the dma_alloc_coherent().

This patch is a workaround for Xen PV.  It consists of the following
changes:
- For Xen PV, use only the fallback allocation without
  dma_alloc_noncontiguous()
- In the fallback allocation, use dma_alloc_coherent();
  the DMA address from dma_alloc_coherent() is returned in get_addr
  ops
- The DMA addresses are stored in an array; the first entry stores the
  number of allocated pages in lower bits, which are referred at
  releasing pages again

Reported-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Fixes: a8d302a0b770 ("ALSA: memalloc: Revive x86-specific WC page allocations again")
Fixes: 9736a325137b ("ALSA: memalloc: Don't fall back for SG-buffer with IOMMU")
Link: https://lore.kernel.org/r/87tu256lqs.wl-tiwai@suse.de
Link: https://lore.kernel.org/r/20230125153104.5527-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

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