vfio: remove useless judgement

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 81597c834158 ("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.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>