vfio: remove useless judgement

In function vfio_dma_do_unmap(), we currently prevent process to unmap
vfio dma region whose mm_struct is different from the vfio_dma->task.
In our virtual machine scenario which is using kvm and qemu, this
judgement stops us from liveupgrading our qemu, which uses fork() &&
exec() to load the new binary but the new process cannot do the
VFIO_IOMMU_UNMAP_DMA action during vm exit because of this judgement.

This judgement is added in commit 8f0d5bb9 ("vfio iommu type1: Add
task structure to vfio_dma") for the security reason. But it seems that
no other task who has no family relationship with old and new process
can get the same vfio_dma struct here for the reason of resource
isolation. So this patch delete it.
Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
Reviewed-by: Jason Gunthorpe <jgg@ziepe.ca>
Link: https://lore.kernel.org/r/20220627035109.73745-1-lizhe.67@bytedance.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_iommu_type1.c

@@ -1377,12 +1377,6 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 
 		if (!iommu->v2 && iova > dma->iova)
 			break;
-		/*
-		 * Task with same address space who mapped this iova range is
-		 * allowed to unmap the iova range.
-		 */
-		if (dma->task->mm != current->mm)
-			break;
 
 		if (invalidate_vaddr) {
 			if (dma->vaddr_invalid) {