ALSA: us122l: Fix pcm_usb_stream mmapping regression

commit ac536a84 upstream.

The pcm_usb_stream plugin requires the mremap explicitly for the read
buffer, as it expands itself once after reading the required size.
But the commit [314e51b9: mm: kill vma flag VM_RESERVED and
mm->reserved_vm counter] converted blindly to a combination of
VM_DONTEXPAND | VM_DONTDUMP like other normal drivers, and this
resulted in the failure of mremap().

For fixing this regression, we need to remove VM_DONTEXPAND for the
read-buffer mmap.
Reported-and-tested-by: James Miller <jamesstewartmiller@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

sound/usb/usx2y/us122l.c

@@ -262,7 +262,9 @@ static int usb_stream_hwdep_mmap(struct snd_hwdep *hw,
 	}
 
 	area->vm_ops = &usb_stream_hwdep_vm_ops;
-	area->vm_flags |= VM_DONTEXPAND | VM_DONTDUMP;
+	area->vm_flags |= VM_DONTDUMP;
+	if (!read)
+		area->vm_flags |= VM_DONTEXPAND;
 	area->vm_private_data = us122l;
 	atomic_inc(&us122l->mmap_count);
 out: