vfio: selftests: fix crash in vfio_dma_mapping_mmio_test

Remove the __iommu_unmap() call on a region that was never mapped.
When __iommu_map() fails (expected for MMIO vaddrs in non-VFIO
modes), the region is not added to the dma_regions list, leaving its
list_head zero-initialized. If the unmap ioctl returns success,
__iommu_unmap() calls list_del_init() on this zeroed node and crashes.

This fixes the iommufd_compat_type1 and iommufd_compat_type1v2
test variants.

Fixes: 080723f4d4c3 ("vfio: selftests: Add vfio_dma_mapping_mmio_test")
Signed-off-by: Alex Mastro <amastro@fb.com>
Reviewed-by: David Matlack <dmatlack@google.com>
Reviewed-by: Yuan Yao <yaoyuan@linux.alibaba.com>
Link: https://lore.kernel.org/r/20260303-fix-mmio-test-v1-1-78b4a9e46a4e@fb.com
Signed-off-by: Alex Williamson <alex@shazbot.org>