iommufd/selftest: Add missing close(mfd) in memfd_mmap()

iommufd/selftest: Add missing close(mfd) in memfd_mmap()

Do not forget to close mfd in the error paths, since none of the callers
would close it when ASSERT_NE(MAP_FAILED, buf) fails.

Fixes: 0bcceb1f51c7 ("iommufd: Selftest coverage for IOMMU_IOAS_MAP_FILE")
Link: https://patch.msgid.link/r/a363a69dbf453d4bc1bde276f3b16778620488e1.1750787928.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>