iommufd/selftest: Add asserts testing global mfd

iommufd/selftest: Add asserts testing global mfd

The mfd and mfd_buffer will be used in the tests directly without an extra
check. Test them in setup_sizes() to ensure they are safe to use.

Fixes: 0bcceb1f51c7 ("iommufd: Selftest coverage for IOMMU_IOAS_MAP_FILE")
Link: https://patch.msgid.link/r/94bdc11d2b6d5db337b1361c5e5fce0ed494bb40.1750787928.git.nicolinc@nvidia.com
Cc: stable@vger.kernel.org
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>