ALSA: us122l: mark hwdep mappings VM_MIXEDMAP

mm has recently moved to a strict model which requires any driver which
clears VMA_MAYWRITE_BIT to be mapping non-mm-managed memory
(VMA_PFNMAP_BIT or VMA_MIXEDMAP_BIT), failing any such mmap() with an
error and a kernel warning - see "mm/vma: only allow mmap to clear
VMA_MAYWRITE_BIT if not mm-managed".

However, simultaneous to this, commit 71c610aeb1 ("ALSA: us122l: Prevent
write upgrades for read mappings") updated us122l to clear
VMA_MAYWRITE_BIT without marking the region with either VMA_PFNMAP_BIT or
VMA_MIXEDMAP_BIT.

vm_insert_pages() and friends set this automatically, however this driver
maps its pre-allocated pages from a fault handler, so never calls these.

Simply set the flag in usb_stream_hwdep_mmap() to resolve the issue.  This
correctly marks the mapping as one whose pages core mm does not manage,
which has no meaningful impact for this driver beyond preventing the
mmap() from failing.

Link: https://lore.kernel.org/20261023-fix-us122l-mixedmap-v1-1-e7bdd053850f@kernel.org
Fixes: 71c610aeb1 ("ALSA: us122l: Prevent write upgrades for read mappings")
Signed-off-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reported-by: David Carlier <devnexen@gmail.com>
Closes: https://lore.kernel.org/20261018025905.201296-1-devnexen@gmail.com
Acked-by: Takashi Iwai <tiwai@suse.de>
Cc: Kazuki Hanai <hnkz.64@gmail.com>
Cc: Jaroslav Kysela <perex@perex.cz>

sound/usb/usx2y/us122l.c

@@ -193,7 +193,7 @@ static int usb_stream_hwdep_mmap(struct snd_hwdep *hw,
 	}
 
 	area->vm_ops = &usb_stream_hwdep_vm_ops;
-	vm_flags_set(area, VM_DONTDUMP);
+	vm_flags_set(area, VM_MIXEDMAP | VM_DONTDUMP);
 	if (!read)
 		vm_flags_set(area, VM_DONTEXPAND);
 	area->vm_private_data = us122l;